Really?
Imagine you're a first level Wizard, and you're chatting up a cute girl (or boy) in the tavern, and you find out that he/she is also a 1st level Wizard, and the DM says that he/she will pay you 25 gold per spell to copy some of your spells. Would you take it?
(The answer might be: "this is sufficiently strange that I think there's a catch, so I refuse". But I would suggest that's only because DMs don't usually initiate interactions unless there is a catch. If this were an accepted part of D&D, instead of an unprecedented event, most people would say yes.)
And if you would let somebody copy your spells for gold, why wouldn't somebody else let you do the same?
And why wouldn't a non-spellcaster who comes into possession of a spellbook start a business "renting" it for this purpose, rather than just selling it?
Again, this depends on wizards being common enough. But I would think even if they are only as common as blacksmiths in 21st century America (as opposed to as common as, say, whale bone corset makers in 21st century America) that would be enough of a market. So in a very low magic setting, no. In everything else, yes.
As I argued elsewhere, sure you could start making up reasons on a case-by-case basis why this wouldn't happen. But it would be a pretty obvious case of the DM saying, "I don't want this to happen so I'm going to make up reasons." It would better (in my opinion) to introduce a mechanic that makes copying spells more complicated/risky than what RAW suggests.
Which would also (again in my opinion) make it more fun. If you find a spellbook it's not just "ok, we both copy all the spells into our books, and then sell it" (which raises the question of why the book is worth money if it's so easy to copy...unless you were starting a business renting it out...etc.). I'd rather you make a dice roll when you copy it, and if you blow the roll you ruin the spell. It just adds more interest to the whole endeavor, and makes spell books that much more valuable.