I get rid of most of them. The buy back table at the bookstore is gravy. I'm very unlikely to crack the book open ever again, so getting a couple bucks, and allowing another student to save some money next semester seems like a good idea to me.
There was one that had some minor water damage. Took it to the buy back table, where the guy paged through it, spotted the water stains, and handed it back to me, saying "we can't buy this." I handed it back to him and walked away. In retrospect, I should have given it to the library, but it was getting late and I didn't feel like walking over there.