Well I don't take issue with your disagreement, butwith RP, you can do what you want.
You do not need a rule because this is a role posture. Even with rules, there are those who respect or not the rule.
If you wanna copy a book you cannot, do it and defend yourself in tribunal.
If you wanna buy a book at a price it is not worth, you can RP it as for example "the stupid magus who buy anything".
Your reason for doing so? you don't need one.
But if you want a reason, you won't find one and thus, the mechanics enter in play, and they don't help you.
Maybe I was unclear, but imo: sound summae are unrealistic because all the book thing in the game is unrealistic. All of this is gameplay.
On that gameplay feature, the RP is provided. You adhere, or you don't. Personally, I don't find interesting.
The order can become a modern-university system with PC intervention. NPC canon are shit (although the latest ones, from Through the aegis and those of Magi of hermes are a lot better than the rest... maybe because the creators of such magi had an idea AND devoted themselves to make those characters "having a soul"... I'm not sure you will understand, but personaly, and that's my current issue, I'm unable to create any character with a soul, because my current magus is the ideal for me. Since I have made it in 2011 and playing him since then, I never had any other desire to play another magus.). NPC books are bad. The order has a lot of magi, but not enough, etc.
Books and trade ... just avoid it, it's my advice. (Personaly, I have not read a book in game since the 10 years post gauntlet, and that's for a 70+ years gauntlet character. And yet, I have the highest arts of all characters due to experimentation [which lead me 3 times in breaking reality adventures, where we had to repair what my character had done... and which was fun]. But not all players will agree.)