I’m lucky in that there’s a used book store right across the street from the library.
So, anything they want (which these days, is maybe 1 book in 20), I get a buck or two for, and the others go across the street.
I tend to think that if it’s a book the book store actually wants, I’d rather sell it to them than give to the library, since that means there’s likely to be someone who really wants that book enough to support the commercial enterprise, but I could be wrong.
Whatever bottom of the used book market that hasn’t already fallen out is going to so in the next ten years as people realize that it’s cheaper to give books away and buy the electronic versions than to move them from place to place and rent space to keep them. Some people will still keep books, but they’re rapidly becoming disposable items.