Go ahead and write him up for the collection thread. With the tweaks I think he will make a good addition and be one more in a style different from my own.
That is the hardest part of making a fair that feels organic for a single writer. You end up with everything in their style and for anyone who has been to an actual fair you know that fairs are actually a hodge podge of many different styles and things.
I certainly agree that a monopoly on library services would be extreme. I do not even see why Durenmar would care, given that there is supposed to be a queue for access.
A more relevant point is how broad a range a single magus, without the history and political clout of Durenmar, could possibly stock. How many scribes could he manage to keep copying? And if he is really good, he would attract so many clients that he could start charging beasts of virtue, or dragon hearts, or something else a lot more valuable than the vis you quote.
But turning it around, all problems are solved with a single stroke. Make a personality which is fun to play and selection of books narrow enough for you to remember it during play, then it is possible to RP the trades, discuss available books, and find common interests IC. At the same time, the operation becomes small enough not to bother the big players.
This is awesome, guys. I don't often read through an entire thread, let alone forward to the rest of Atlas' staff, but this one is exceptional. Ars Magica goodness galore! 
I'll try another. I'm still doing the basic bones of the concept and letting any prospective SG flesh it out a bit.
Miratius of Tytalus does not have a stall. If you do not know who he is, then clearly you are not worth his time. The service he offers is to cast spells which improve characteristics. The cost I suggest is double the vis cost to cast the spell.
Is the cost high for something that literally takes 3 hours? Maybe, however, the PC is not paying for the 3 hours to cast the spell, they are paying for the 20+ years of study to be able to cast these spells.
As Miratius needs to be a CrCo specialist, he can offer healing for a fee if the SG wants to add another string to his bow.
Miratius believes the most important task for any being is to improve themselves. In his own life Miratius has taken this to the extreme. Miratius is physically and mentally close to the pinnacle of what is humanly possible. This has been achieved by an extreme fitness regime, intense study, and multiple Creo Corpus and Creo Mentem spells to complete his quest for perfection.
Miratius looks like the classical sculptures. He is incredibly toned, clean shaven, well groomed, and at most times is shirtless to show off his physique. Miratius is vain and arrogant to the degree he does not think he is better than everyone, he "knows" he is better than everyone. Part of the reason he has healing spells is he will not tolerate being weak. Light wounds, he'll shrug off, and let them heal with assistance from Purification of the Festering Wounds. Anything more is not acceptable.
Miratius will be patronising and critical of anyone asking for his services. If someone disrespects Miratius in any way, he will not cast the spell they want without a grovelling apology. Miratius will not bargain. It doesn't matter if the player rolls a 24 bargain check, Miratius does not bargain!
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Miratius is very quick to consider something being disrespectful. For example: If a player questioned the cost, arguing a lower level cheaper vis spell would achieve the same outcome, Miratius would tell them to cast the spell themselves.
As Miratius hates customers so much, why does he sell his services? He needs the vis. His self-improvement, from sourcing the summae and tractatus to improve his Creo, Corpus and Mentem, as well as lab texts for the improement spells, they all cost.
Miratius honestly thinks his customers are pathetic for not improving themselves and won't hide it. He literally despises anyone who in his eyes is weak, no matter the reason. It is this part of his personality which explains why he hasn't improved his communication. He loves being patronising and arrogant, and is concerned his personality will change too much if he magically improves his communication.
He is written to be an eminently unlikable person. Due to his personality, there is a question of who would buy his services? What he is offering is something only a select amount of magi can do, and most who can cast these spells would be a lot older, with a conflicting demand to make longevity potions. His niche service suggests some magi would tolerate him as they really want to buy what he is offering.
I've included some stats, however it's not a full write up as I'll let anyone using him flesh out the last parts. .
Characteristics: Int +5 Per +2 Str +2, Stam +5 Prs +4 Com 0 Dex +1 Qu +2 (Int, Per and Prs improved from +3 since gauntlet)
Age - 45 (can easily have his Age increased, with a commensurate increase in his spell selection, arts and stats. 45 is arguably the minimum age to achieve what is required.)
Essential Virtues/Flaws: Improved Characteristics x 3, Affinity Creo & Corpus. Puissant Creo, Major Magical Focus - improving people. Arrogant/Proud (-3), Restriction (can not be voluntarily holding or wearing magical items).
"only those whose magic is not strong enough depend on props". The restriction is purely psychological, however, he would prefer to die than to be seen to be reliant on a prop.
Traits: Arrogant +3, Vain +3, Driven/Focused +3.
Abilities: Parma Majica 5, Magic Theory 4, Latin 4, Artes Liberales 1, Concentration 1, Finesse 1, Penetration 1, Order of Hermes Lore 1, Native Language (nothing ties Miratius to any cultural group, so choose what works for your story) 5, Athletics 3, Brawl 3, Survival 2, Swim 2.
His abilities are relatively low, apart from Parma, due to his focus on driving up Cr, Co and Me.
Arts: Cr: 20(23) In 6 Mu 6 Pe 6 Re 6 An 6 Aq 6 Au 6 Co 17 He 6 Ig 10 Im 6 Me 12 Te 6 Vi 6.
Essential Spells Known
The enduring mage (lvl 60) Touch range +1 permanent stam increase +5 max, The genius mage lvl 60 int version, The imposing mage lvl 60 presence version.
Pilum of Fire mastery level 2.
Variant Panic of the trembling heart (range sight, target group, duration ring. any rational mage should have a crowd control spell)
Seven League Stride
Purification of the Festering Wound
Incantation of the Body Made Whole
Restoration of the Defiled body.
Miratius could have all of the other +5 max characteristic boost spells or none. For the sake of saving a few vis, he will not learn the lesser versions of the characteristic boost spells (or lesser healing spells for that matter). The season spent learning that spell in not worth the future small vis savings. Long term, it's a smart business move as he can sell his services cheaper, and make the same or more vis, however, Miratius is a reluctant businessman and doesn't consider those things.
Potential interactions with PCs
Buying the spells.
If a SG wants a wizard war, Miratius could easily be baited in to one.
Miratius is all about self reliance, so he won't send the PCs on a task, however, the PCs could hire him. He wouldn't come cheap, however he would be useful.
Marcus of House Tytalus, Archmage Loser Extraordinaire, having lost 7 challenges.
Marcus is a short but handsome and cunning man. His age is indeterminate, sometimes he seems in his early 20s, others, late 50s. Rumors are this is a result of twilight episodes. His plots, both successful and unsuccessful, are known throughout the order for being grandiose and skirting the Code. He loves to talk, especially to attractive young women, his one weakness. One of the few open Titanoi, he bargains with spirits for power, and thus has the eye of the Quaesitors on him.
At the tribunal or fair, Marcus will be setup from an enchanted box that with the correct command, springs open into a full sized store.
In this, magi will find many odd knick-knacks and petty enchantments. These are often simple, low level but obscure effects, such as ointment that repels the interest of blonde Badgers. The true value of his store is rarified items such as the first exhale of a fae blooded baby boy, the steam of a fire elemental thrown into the North Sea, dabs of the rare times snow fell in the Sahara desert. Such items are not typically vis, but appealing to certain types of Fae, Magic spirits, and possibly even demons.
Marcus is rumored to have other, darker items such as the blood of Dark Fae, or even Demons. Quaesitors have tried to March him twice for selling cursed enchantments, which Marcus does in fact have from time to time, but he only sells to the right kind of Magus.
Vis is commonly accepted, but selling Marcus other rarified, unique things of interest to Fae are a sure way to get into his good graces.
Unfortunately for the PCs, the vague rumors around him are true. He has recently slipped into Diabolism, but thinks he can still keep his soul.
Adventure seeds:
(1) The PC's need an obscure item in order for forge a deal with Fae, Marcus' stall is one such place they will find what they need.
(2) Quaesitors ask the PCs to help them collect evidence against Marcus. Possibly unknown to the PCs, the Quaesitors have tried this before and failed, such helpful magi disappearing.
(3) An angry spirit of some sort, feeling abused by Marcus in one his plots, approaches the PCs to help exact its revenge against Marcus.
(4) The PCs are approached by Marcus to participate in one of his plots. He will offer to pay them in his rarified materials, which they desperately need. Unfortunately for them, the plot is against an ally of theirs.
Another story seed: if a player character is a good writer or a combat magus, Ferunius asks them to write a book or lab text or book on spell mastery in their area of specialty, so that it can be distributed around the Order. While they would prefer people to do this for ideological reasons, they may be able to pay for the privilege.
Writing for Ferunius may attract political sanction.
In the mundane entertainment section: travelling players, doing mystery plays about significant events in the history of the Order (the founding, the Schism War, St Nerius etc).
Possibly they're Redcaps, possibly they just mundanes with Hermetic connections. They only get to do magical plays at Tribunals or other Hermetic events, so most of the time they're on the mundane fair circuit, doing mundane stories. They attend Tribunals because the audience pays well, and maybe someone needs a longevity potion.
Adventure seeds:
I would like to expand that great idea of this travelling Troupe! 
I can imagine that such a Troupe is financed by House Tremere for playing a moral story about the Schism War and how the Tremere (and Flamboue) saved the whole Order with their heroism. Naturally they play other scripts as well to "mask" the real purpose of being propaganda.
While they are playing the famous moral play different mundane butlers/helpers of seemingly independent magi starts booing and shouting. The whole situation turns out a mess and the chief quesitor starts an investigation bc of the scandal. Rumor has it that all the servants were influenced by magic, but the quesitors cannot grasp the perpetrator.
The players catch a semi-secret conversation about the scandal - one of the magi obviously enjoying the scandal connected to the Tremere - and one player can identify the sigil of an infamous Tytalus mage when he leaves the site of conversation, but it is also cler that he was in disguise (Persona). He is the mastermind behind the scandal implied by the conversation.
If the players ask the Tytalus mage about the scandal he will call them to a secret meeting and presents them the information about the true happenings of the Schism War and seemingly credible evidence that the Tremere orchestrated the war bc they were afraid of the Diedne's numbers and wanted to restore some balance to the numbers. (They knew very well how dangerous could be if somebody has the numbers to give a try contorlling the Order. 
)
The Tytalus hopes for getting help from the players to clarify the real bad boys behind the Schism War. But is it not rocket science that this kind of political game could easily lead to another war in the Order or high level assasination and political machinations.
The Tytalus seems to be adamant: his only goal is prepare the Order to accept the grim truth and when the time comes present it to the Order.
The troupe being paid to promote propaganda is a good story option, but there's no Hermetic crime in people booing a play. Not even a hint of one. Even if they're influenced by magic. While PC magi may wish to investigate, it would be a hard sell to get Quasitores officially involved.
You do not need a crime to make a war.
You know, there is the Code and there are the different factions. Just look at the Constituiton of the US for Ex. It should be a clear set of rules.
But if you are on one side, you consider the rules supporting your wants and needs, and if you are on the other side you see the rules as supportive to your clearly conflicting wants/needs as your opposing faction seems them.
So I can easily imagine a situation when you as a Quesitor start an investigation to cool the mood. You are totally aware of the potential outcome of big fat nothing, but it is a good tool to deescalate a situation. Like when the police IRL says : "We will conduct a comprehensive investigation and we ask everybody to be patient while we are doing our job!"
This small stall is occupied by a single maga in a purple robe. Apart from the maga the entire stall is full of dried fruits, seeds, pods, bags and jars all containing what appears to be more fruits and seeds, as well as jars of jam, jugs with exotic wines, sweets and any other imaginable fruit product. None of the fruits look like any known fruit, nor do the seeds look like anything recognizable, though on closer inspection many of them appear to slightly resemble the fruiting bodies of known plants.
There are weirdly familiar but exotic looking potted plants in the stall: Stinging nettles bearing apple sized fuzzy looking fruits, Heather bearing stand of grape-like fruits, A small juniper with juicy looking berries etc.
The maga claims to be able to cause any plant to create fruits and she is willing to sell the seeds necessary to plant these supernatural plants. The wares including things like acornfruit jam, lindenberry wine, basically anything fruit derived product you can imagine, but from a plant that doesnt normally bear fruit.
For a bigger fee the maga will even travel to nearby covenants and cast the spell on plants that the covenant desires, in case she hasnt thought to create a fruit-bearing version of whatever is their favorite plant. Though she has no control over the appearance or flavor of the new fruit.
The backstory here is that the maga has recently invented a version of the CrHe spell that causes a plant to grow to full maturity in the span of a single day/night. However because she invented the spell using experimentation she ended up with an interesting side effect: The targeted plant will always bear an edible fruit once the spell has run its course, and the plant will be forever changed such that it bears these fruits instead of whatever fruit it normally bears.
Any input on what this needs in terms of correction or fleshing out in order to be publishable on the proper list?
A rough write-up of the Magi. This could be just a paragraph description of appearance and personality (what I have been doing). It does not need to be heavy, just give an SG something to go off of when running interactions at the shop. How do you envision them being portrayed. Right now the only thing we have to go off of is they wear a purple robe and created a spell by experimentation.
A short blurb for each of the major products/product categories/services that they are selling. You have much of this for several already as product categories (Fruits, Plants, Seeds). Maybe come up with a single 'specific fruit product' to help jump-start an SG's imagination. You hint at some (acornfruit jam, lindenberry wine) so pick one and give it a paragraph blurb in the products section with things like appearance, flavor, scent, and any effects.
Are they offering all of their products to everyone? If not then what do others need to do to be able to buy certain ones needs to be in the individual product description. This could be as simple as being friendly and engaging with them (I don't sell this to donkey wells), some challenge (eat a hot pepper), etc etc.
Overall it looks good.
Really like the play suggestions for her. She could be a very memorable encounter.
I really like this post-a-day from @raccoonmask and think it would be right at home at a Tribunal Fair. I won’t replicate the whole post, but Akakios offers the unusual bargains you’d expect from a faerie — trading your destiny for a stone of luck, or your fears for a gauntlet of might; with mechanics to match.
I think some of the bargains might have house rules, but nevertheless it’s good food for thought.
Another
Information and Complaints Booth.
The House of Guernicus knows when there is trade, there is the chance of scams, theft, and claims of wrongdoing. The information and Complaints booth is an attempt to minimise this.
The magi running the booth constantly changes and is always young. House Guernicus wont send the equivalent of a supreme court judge to small claims court. This booth is seen as a good starting point for young Quaesitors to get some experience.
It is a small booth with books containing commentary on other booths supplied by previous customers. Anyone can submit a testimonial, however, the Quaesitor will vet the testimonial for accuracy, and if it is accurate, will add the testimonial to the books.
The only consideration is accuracy. To use some of the previous entries as an example.
Comments on Quaesitors trying to march Marcus is fact and would be in the booth, rumours of his Diabolism can't be proven thus wont be in the testimonials. Comments by customer who were greatly annoyed my Miratius's service will be there.
While the testimonials must be accurate, the person at the booth is often happy to gossip and indulge in idle speculation.
Prior to the fair starting, the Quaesitor will investigate every booth for any diabolic influences, potential magical or mundane traps, and basically anything which might assist in plans to indulge in foul play during the fair.
The Quaesitor will settle disputes. To stop frivolous claims, any person submitting a dispute must offer up 1 vis. If the Quaesitor determines the dispute was frivolous, the 1 pawn of vis will be surrendered to the subject of the complaint.
The Quaesitor being young, is given a few items to assist investigations. An item which can identify curses and diabolic auras. An item which can cast frosty breath of the spoken lie with good penetration. An item which can send a message to a senior Quaesitor, who will use the leap of homecoming to arrive at the site. It is generally seen as bad form to call the senior Quaesitor, so this is only done in extreme cases.
Potential character interactions
Depending on the nature of the investigation I dont see magi willingly submit to an inspection by a quaesitor on a justification as flimsy as "I might find something on some of you if I investigate all of you".
There would clearly be a jurisdiction issue, however, those who refused; it will be reported they refused their booth being inspected.
Who knows, it might draw some people to a certain booth, knowing they wont bow down to "the man".
You should consider that the whole Order is about giving the freedom and the right to be not bothered to the society of magi. This kind of "you are guilty if you do not cooperate" mindset is nice for the inquisition, but not for magi, and actually against the spirit of the Code.
We have too little Tytalus causing havoc at this fair. =]
There is some handwaving in the end, but the first part should be usable by anyone.
Type: Entertainment
Description: A magus sits in a chair at the middle of an open space. At his side a sign reads "challenge me". There is no booth.
Background: Erastrus is keen to hone his debating abilities. He invites any passerby, magi or not, to an argument, just for the sake of arguing (Tip: for bonus points, change his name from Erastrus to Barnard). Things like your opinion on yesterday's supper, or the clothes he is wearing. He is also open to debating specific fields, such as the latest developments of Magical Theory or the true nature of angels.
Rules:
Topics for Discussion: If pure discussion, anything. One favorite of Erastrus is arguing that there is a booth (either an invisible one or a metaphorical one), while the challenger claims there isn't. Another favorite is to claim they are (or are not) having an argument. For academic discussion, the abilities for the disputatio should be expanded to include Magic Theory, Realm Lore, Code of Hermes, or even Arts.
Winner: It doesn't matter! Arguing is fun! But Erastrus is willing to bet a pawn against certain selected opponents.
Erastrus stats To present a better challenge Erastrus should have 5 in all of Folk Ken, Artes Liberales, Charm, Guile and Leadership, and at least a 7 in Intrigue, which means he is at least 10 years out of gauntlet. He prefers a balanced approach to debating, with no weak abilities.
The Plot Thickens
(NOTE: Everything below can be safely ignored. There is some degree of improvisation in it, and it's not really necessary for the Eristic Moot Booth to be interesting. If desired you can also just handwave everything below.)
Erastrus uses this debates to subtly probe the challenger on his position concerning certain social topics in a hidden way. For example, inside what seems like a theological discussion (academic or not) about Mc 12,13-17 (Render to Caesar the things that are Caesar’s, and to God the things that are God’s) Erastrus might be actually probing the challenger on his stance about House Bonisagus claiming other people apprentices. Hidden in a debate about "is butter better than milk?" he might be ascertaining what would be your position on the topic "is a magi better than an apprentice?".
Probing: When Erastrus is probing someone his arguments become weaker (less logical, more easily counteracted). He isn't even really trying to defend his point. Erastrus starts the discussion with a discussion fatigue level (see debate/disputatio rules).
Discerning the enemy: At the end of the debate, roll Com + Intrigue for Erastrus. On a 12+ Erastrus determines what is the political position of the challenger on the hidden subject. Optionally, add the argument strength to Erastrus roll (it doesn't matter if he won or lost, he gets a positive bonus).
Noticing the deception: if an observer (including the challenger) succeeds on a Per+Intrigue check against Erastrus Com+Guile by 3+ he realizes there is something going on. By 6+ he discerns the hidden topic. If the challenger notices that something is going on he can choose to lose/end the debate (usually in a pathetic way). If he realizes the hidden topic he can try to reveal nothing (-3 for the debate totals) or to mislead Erastrus (-6 for the debate and -3 for the guile check, since it will be easier for Erastrus to spot inconsistences). Either way he gains a Com+Guile roll against Erastrus Com+Intrigue (and the table on the right column of HoH:TL p. 24 should then be used).
What is in it for Erastrus: First and foremost, Erastrus thinks it is fun to force people do divulge positions that they themselves might even be unaware of. He usually sells the information he gets. Eg., if he learns someone is against a certain move from Durenmar he can either sell this info to Fengheld/Dankmar or to Durenmar. Sometimes he will sell the info to both sides. Most info is traded for other info, or favors.