Sure, don't see why not.Can two PC wizards copy all the spells in a looted spell book? And then sell it?
Heck yes. Spells are power. Power can be used against you. Why would you take that risk with a random stranger, or even a passing acquaintance? Besides, why give something away for free when you can demand a price? There ain't no BitTorrent in D&D.Is there any reason a friendly wizard wouldn't say, "Sure! Copy all my spells!"
At the very least, any NPC wizard ought to demand a trade: For every spell you copy out of my book, I want one from yours that I don't already know. And even so, NPCs might be reluctant to part with spells that could easily be turned against them. If I have a tower shrouded in magical wards, I'm not going to go around teaching people how to cast dispel magic.
As for a college of mages allowing other wizards to copy spells from its library, that seems highly unlikely to me. Knowledge is a college's stock in trade, and those spells are the most valuable knowledge in the college's possession. If you're not a member of the college (and that ain't an easy thing to become), they're not going to let you walk off with their most closely held secrets. You'd better be prepared either to pay through the nose, or to trade a new spell that isn't in the college's library... and finding such a spell will be difficult to say the least.
Living in the Internet age, I think we tend to forget how valuable information can be when it can't be replicated across the planet at the speed of light.
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