Never sell them back to these fucking assholes who fuck you over when you buy then, then fuck you over again when you sell them back.
Sell them directly to someone else. I would always list my sells for $1 less than whatever was the lowest price anywhere was selling them, guaranteeing anything I sold would get bought literally a minute or two later. You can make more by choosing some optimized price using some math formulas you can infer from the distribution of prices, but fuck that. I like rewarding the kind of people who actually sit there refreshing and then just immediately buying from whoever is offering the cheapest price. Those people are my buds.
And for some reason, there are always a couple weird, bizarre places and specific dumpsters that end up with giant piles of books in them around medical and law libraries in big cities. Do these people somehow get together and agree that they're just going to abandon piles of $100 bills in random study locations and dumpsters? I don't know. But sometimes you'll find huge hoards of books most of which are still being used and sell them for $50 apiece.