Selling Magic Items

Question for you GMs out there - Have you ever let your PCs sell magical weapons? What about spell books? I don’t think this is too rare; I think it is common in most games. What happens to those items? If the party fights a demon summoner and finds his spell book, they would most likely sell it if they couldn’t use it (and maybe if they could). So some guy buys the book (and probably kills the middle man). He is now rampaging in the PCs homeland with demons. They put him down and get the book again. They’re stupid - they sell it again, of course, since the last guy is now dead, no one will pay well for it because they understand there is danger. So they sell it to a “passing merchant in a bar”. He either is or off loads it to a truly dangerous man who summons the demons as part of a major assault on the government of the city. #1 - the players seem to be endangering themselves needlessly. #2 - Shouldn’t someone (like someone afraid to buy the book) tell the government that the party is at fault for the demon invasion?
OK - go the other way - They dump a whole bunch of magic items on some merchant and he sells them to whoever he can - like the bandits who the players will be fighting next. Does the party just keep arming their opponents? At some level they will think how smart they are for selling the same items over and over again, but that would imply that you aren’t making the missions hard enough.
A while back in one of my campaigns a vorpal claymore came home from a mission. Most of the party was quick to try and sell it, but a couple of them said, “No way! I’m not going against someone with a vorpal claymore!” Funny how the melee types did not want to sell it, but the mage types weren’t worried. Hmm, there may be a moral in that somewhere.
Even fantasy characters have to learn - there are consequences to every action.