When Redcaps arrive at my player's covenant, they are always eager to trade vis (they are vis rich in some flavors and vis poor in others) and ask after new texts or trades are on offer from other covenants.
Does anyone have a good system or suggestions for deciding what's available?
I'm working on a set of random tables currently to assist me, but if some of this work has already been done then I would welcome someone saving me the effort.
I have a system for PCs trading but only when they are the seller. (it's very complex - i redacted an explanation but it is still 15 word pages...) and that doesn't answer your question),
Otherwise, IMO, a good idea is to look online games and see what they have as labtexts, do collection of it... Besides it's feeling of "reality", it provides interaction with covenants having an history and not just "XX covenant".
For example, in an online saga, they are selling that material and urgently need vis.* Their Jerbiton magus is currently selling those in Scottland and HIbernia (but that doesn't really matter since you should adapt it to your saga ^^)
Nothing more fun that joining history with other PCs covenants and including names of existing people. 
.For vis, I would say everything is always available (well, unless your players start asking for 100 pawns of one Art). That's why people pay the 2:1 exchange rate, to pay the Mercere's efforts at making vis available by moving it around. As a surprise, you could have the redcap inquire about vis of a random Art and offering a slightly better rate, mirroring some other covenant having a great need of it.
For texts and trades, I would say well established covenants have no need to initiate such trade, so only spring covenants are actually offering through the redcaps, and nothing fabulous: the players will have to inquire (through the redcaps if they wish) after specific texts if they want them.
Summa: highly unlikely, as a rule, unless someone has written a vanity book that is a bit shit and is looking to cash in. Summa trade would typically be between covenants - and even then is probably done on a loan basis rather than a trade basis.
Tractatus: All magi can always use more tractatus on everything. Offering up tractatus for trade and expecting them to be on offer is not unreasonable. Most should be of average quality (5-7) with correspondingly higher prices for more impressive ones. If the players have someone internally who can write q11+ tractatus, this can be a very lucrative trade source.
I figure that trading tractatus for lab texts is quite reasonable, but only if the person is requesting those lab texts. Offering a lab text of something basic (here's my lab text on Aegis level 20!) is probably not going to net you much because everyone has it.
Magic items: Making magic items on commission is a great way to score some vis. A trade in common magic items is not unreasonable to expect: the mercere commission such common items with the intent of on-selling for profit. We run with the mercere providing necessary vis and paying cost x 0.5 to the magus doing the work, and on-selling at vis cost x 2 to the buyer. Common items include things like magic lamps, basic travel devices, items for easy communication, etc. Weirder and more esoteric items are a case of commission someone specifically. Order of Hermes Lore roll to work out a suitable contractor, then write them a letter and negotiate from there.
Service: Some magi want stuff they can't make themselves. The players may want to buy service (longevity potion by creo corpus specialist with minor focus in aging, for example) or be well known enough in the Order that they are approached with requests. Payment for such services can take the form of any of these items, or straight-up vis.
Interestingly enough I was considering writing a post fora different thread [url]https://forum.atlas-games.com/t/what-would-you-want-in-a-5th-edition-grimoire/5810/1] saying that I would love to see a nice treatment of Hermetic Economics in a supplement. Something written to help troupes handle trades like these consistently but with an eye on the needs of the individual saga. I just wasn't sure if this was the sort of thing anyone else might be interested in seeing.
One idea I once proposed on the forum was that each of should post a list of what their covenant was willing to buy and willing to sell. This would give everyone a good idea of what's on the market. For some reason the idea did not catch as much as I had hoped ... but may be it's time to try again?
Our (late summer) covenant seeks:
Seasons of service from other magi - particularly help in the lab (high intelligence and magic theory preferred!)
Magical items, in particular: 1) lesser enchanted devices with some simple, specific effects and b) anything non-hermetic, regardless of utility.
Tribunal votes on various issues.
Vis: in particular Creo, Intellego, Rego, Animal, Corpus, Mentem, but we are willing to buy anything for the right price.
Gifted apprentices.
Excellent books (quality 11+ at a minimum).
Information on any exotic magic.
We are willing to provide:
Ezzelino, let's try to play this 
:
Where is your covenant, are the magi willing to provide arcane connection for teleportation by Redcaps and how much time do you need the magus? One of the magi in Zapad Na Vostoke could help. (I obviously do not take our timeline into account since we are currently in autumn 1215) But even the maga is a very humble labrat which is willing to help sodales, she is quite skilled in vis, and any offer has to exceed her vis extraction ability to be considered (vis extraction is currently 13p per season). She provides +23 (MT+int). Of course, she is not willing to help too young magi and reserve her skills for middle/aged one.
Zapad offers many lightning items.
Maybe a possible trade here. See our wiki.
Zapad needs silver and vis !
Exarkun:
We provide transportation via flying ship, covering about 500 miles/day - this means less than a week to cover the width of Mythic Europe.
14+ pawns is steep, but not unacceptable for 1 season of +23 bonus in the lab. We'd probably be willing to pay 14 pawns or so of Herbam or Aquam, or 10 of Perdo (which is more valuable than Vim vis, because it's a Technique). Depending on the circumstances, we might go as high as 21 pawns of Aquam or Herbam, or 15 of Perdo once in a decade (but then you must be available on a very, very specific season, no haggling on that).
I've given a cursory look at your books, and there may be a few we'd be willing to "trade" for similar ones (remember, it's not exactly a trade, effectively we are lending you a copy for as long as you wish, so you can not make further copies nor sell it, lend it, etc.).
Silver : better than that, we can deliver goods at your doorstep, including food, ink etc. Remember that injecting too much silver into the local economy can make inflation explode - we help you avoid that 
Would you be willing to sell vis for silver (or, rather, for mundane goods of your choice)? Would you be willing to sell books?
We are not interested in lightning items. We are either interested in exotic magic, or in a few very specific hermetic enchantments we've never had the time to make and that would probably have to be custom-made.
Helas, not a good idea at all. This maga developed teleportation spell for a good reason: she has motion sickness! 
Deal (perdo vim is fine). Send your payment, season needed and vis pawns to Zapad Na Vostoke, Keythleen Bonisagi, as well as an hermetic contract in good form to be signed in front of our redcap, before delivery of payment, and she will come at the due time. 
. If you let at least 1 year to prepare before invitation, 10 perdo is fine; if you ask it just a season before, 15 is more accurate.
same here: our RAW cow and calf prevent : copying, selling, trading, giving the book. In fact, only the "buying" covenant members can read it. This restriction ensure that authors still write books for others. The same would go for our books.
In fact that should be a question for the council. I'll use the NPC to ask next reunion. May be in a few time IRL since now it's adventure time... We need silver for labs in fact, so if you can deliver lab material, it's fine; but it's also for our cash reserve.
My players wanted to do a permanent Aquam (we have a source ) vs Vim (we have no source! a shame). It can be up to 4 pawns of aq vs 4 pawns of vim per year. Send a hermetic contract for acceptance to Procellus of Jerbiton, de facto Mercantes of the covenant .
Our gruagach don't seem willing to share his knowledge. Helas, on that point, I can't be of more help