Great idea. I have not done this in a while, but paperbackswap was a valuable website for trading.
I love the used book sales! In addition to their semi-annual book sale, my library finally started keeping a few display stands of a couple hundred or so used books right by the entrance on a permanent basis. There are garage sale-type stickers on each book with the cheapo price, and there’s a collection box attached to one of the display stands. The money raised goes to our “Friends of the Library” organization. Sometimes I just end up buying a book and leaving without checking out one. When I finish reading the used book, I just return it to the library via the drive-thru drop box outside so that they can sell it again. I sometimes throw in other random books I have around the house because I want to free up some space at home for more crap, plus I’m pretty sure the library will appreciate my generous donation. Turns out they don’t. There’s now a sign posted on the book drop box for people to please only place actual library books in there and to take donations inside to a librarian. How rude. Lol. Not gonna happen!