How much, and what kind of trade goes between tribunals? I'm specifically thinking of the western-most covenants of the Rhine Tribunal (about 100mi west of Durenmar) and Normandy, and predominantly in terms of enchanted items. I know that Redcaps are important as far as being the go-betweens, and agreements with any Verditius magi in that region as the producers will be vital (Verditius PC wants his covenant to be a hub of trade with them); but I'm not sure of the kinds of things that need to be accounted for in such a venture.
First off: I imagine that there's an inter-Tribunal network of Hermes' Portals maintained by the magi Mercere. So the big hubs are probably the Mercer Houses.
I have the impression that it's not just border trade. There seems to be trade across the continent among covenants.
I imagine that lesser enchanted items, books, lab texts, and a certain amount of vis are the main trade items. There's also rare equipment and ingredients to consider.
There's probably also quite a bit of trade in services, both magical and covenfolk. Covencraft?
I'd expect any covenant looking to engage in trade has the exact same problems as a modern business.
having something to offer for trade is fine, but you need to actually let your potential market know you exist.
Most magi looking to trade probably conduct such affairs at tribunal, because it's a handy place where a bunch of potential customers are gathered together. The alternative is to ask a redcap to mediate; redcap mediation may well go beyond tribunal borders, but it's going to incur some redcap expenses if the redcap is actually drumming up the client base
you have to have something that more local competition can't offer
As soon as you start a trade business that's going long-distance, other magi (esp. Verditius) are probably going to try and muscle in on your action. Your potential clients will be after convenience; they're wizards, and wizards have a tendency to take the path of least resistance on such matters. It gets worse if you are Verditius; they act like a guild, and guilds have fairly specific clauses about competing with fellow guild-members. The kind that may well involve wizard's war or vendetta.
you have to keep in touch with your client base
The moment you go quiet, your client base are probably going to forget you exist. Your only hope here is a redcap who touts your services, but we're back into redcap expenses territory again. The only way to keep this alive is to keep making noise; build that reputation!
international tax laws contain traps for the unwary
Crossing tribunal borders is going to open up interesting potential legal cases. You are going to have no representation worth a damn in their tribunal, and they will have friends. So if a legal case comes up chances are you're going to be the at-fault party, even if you're not. If the business was conducted in their tribunal, chances are the case will be heard in their tribunal. If you're thinking that you'll be fine if you use selling agents, what do you do when the magi from two different tribunals both demand your attendance to have a case heard against you for fraud? At the exact same time, in two completely different locations.
What this says is: no business without taking a season or two out to go and find some (stories), and once found then business [strike]may[/strike] will incur complications (more stories).
If the character has an enemy or a vendetta, this is the perfect place for it to bite.
Now with all this said, the other thing is that there's really very little in the way of legal protection for trade beyond a very terse statement in the Code (thou shalt not deprive a magus of his magical power). This has so many unpleasant ways to be (mis)interpreted. Furthermore, wizard's war exists as a perfectly viable out-clause from the Code's protection; disgruntled customers or competing traders can very literally come over to your house and break your fingers.
In short, I expect Hermetic inter-tribunal trade to be fairly similar to what happens when a Columbian drug cartel starts trading in Mafia-controlled New York. And as for establishing a trade hub - if it isn't waving a Mercere banner, I can think of one group of magi who might object to your undercutting their profits. After all, they provide a valuable service to the Order...
There's supposed to be two big Hermetic fairs, one in the west in the Alps and one in the east in Thebes. I would imagine that a lot of trade between covenants goes on at these fairs. City and Guild probably has more information on this topic.
Since Tribunals are not countries and I don't recall specific trade law ruling inter-convenant and even less so inter-tribunal exchange, my view is that there is no trade as we see now between countries.
I would see much more an opportunistic and ad-hoc business model run mostly by Mercere and to some degree by Verditius. With the exception of business oriented mages (essentially in two-said Houses plus Jerbiton, and possibly Tremere), I don't see mage managing trades - too much distraction from the real interesting stuff (in their eyes). Even less with mundanes considering the Gift issue.
So I don't see chart or trade agreements such as "Novgorod will sent 200 paws every Tribunal to Roman Tribunal in exchange for XXX supply of lab equipments, books and what not".
More like: "Dammit ! we are short on Terram virtus for our new expansion. Okay, everybody toss a decent labtext by next season and I will contact this young Spring covenant to trade that for the few pawns we are missing" (it is my own biase, but I like to picture magi as so engrossed in their research that they are completely lacking in planning and management skills - but that just me, too much Terry Pratchett and Unseen University).
All in all, I see hermetic trade more on individual basis and between covenant that big business with economy of scale and such. Not that there is no opportunity to the entrepreneur-magus, just that's a distraction from lab time.
As an counter-example, in my Saga (based in Novgorod), because of distance, lack of good roads and abundance of bandits, delivery are slow. So my players are slowly setting a distribution center more or less subcontracted by Redcaps for everything which is not confidential (kind of Lab'R'Us merge with Amazon). They get a decent cut, they have enough work to keep busy good craftsmen fulltime and since their product never reach mundane market, they have managed so far to dodge guilds interference.
You send a representative to one, and attend the other yourself? Shows which one you value more, admittedly, but is less of a snub than not showing at all.