Excellent question.
I have similar observations as yourself, myself been struggling a bit to handle this in my own long running campaign (quite entreprenurial players, session 100 next week! The full 500+ page chronicle is here: Ars Magica: The Rhineland Chronicle of Collem Leonis).
The players have taken every chance to fill out their library, beg or trade for more. They've sorted out two scribes (one is lost) and I had to "artificially" limit their successes here.
I have had to go beyond the rules to limit (and even trap) books and trade. For me, Ars Magica RAW is a bit problematic here. In the end we did have conversations on what we want the game to be and players had to agree to scale back exploitation.
A couple of things or house rules I have been pondering:
- Always require (both scribes and magi) to actually have magic theory enough to cover the magnitude involved (or half of it, depending..). Toying with this idea for another campaign to limit spell level you can even learn/use.
- Perhaps allow them to copy with a MT that's not enough, but add a "corruption" type function to works. Perhaps you can copy the latin right, but figures, diagrams, and trickier special notation is just more difficult (could require Artes Liberales at certain level as well).
- Simply limit to one scribe for the covenant.
- Scale back desire to share and learn - especially by the Bonisagi (I have in my campaign)
I guess one could just rule out scribes and require magi - but I'd prefer to put a better limit on them.
What I have done in the campaign though is that generally everything in core rulebook and a couple of the supplements is readily available and tradable. That's kinda where modern House Bonisagus are at. Anything else, magi essentially guard as trade secrets. Also the elder Bonisagi "hold" their stuff for years/decades before fully shared (but they like to style it at Tribunals).
This in effect means that market for core book spells, all summae for arts (to level 15-ish) and many abilities up to level 5 are basically saturated market. i.e. can't really make money to sell, but does cost to buy or get copies of.
Nobody is really interested in lower level tractatus or most usual spells either.
My current problem though is I have some specialists among my players including a Creo Corpus "healer" who can basically get 100+ lab totals now (damn virtues). There's a hilarious account of him dominating the spell competition at the tribunal in I believe session #97 with a level 50 healing spell, including demonstrating charged item effects penetrating Durenmars aegis (level 60) just to show off. And now he's selling those for vis... (argh, but kinda fun). As well as leveraging trading and selling the spell book (in the end we agreed it was better to give it for prestige, as he wants to become a magister).