I usually just sell back to Titles. I often get 50% of the initial price I paid for the book, and that's good enough for me. I can't be bothered spending the time to post ads, meet 5 different people who each want to buy one textbook, etc.
If Titles isn't offering a buyback on a book (common for my program because the profs change books like... EVERY term in some cases), I take it to Darien Lake on the May 24 weekend and use it as fuel for the camp fire.