Luckily, there are alternatives. For example, at Powell’s, you enter the ISBN and the condition of the book(s), and you get back a quote. You then mail the books to them and reap your reward via PayPal. (Although it may not be worth the trouble — for a like-new hard copy of the 2010 novel All Clear by Connie Willis, I was offered $1.67.) AbeBooks is also a popular place to look for used books, but it deals mainly with professional sellers; fees for selling start at $25 a month.
If you have recent textbooks you don’t need anymore — and we all know how expensive those can get — you can try GoTextbooks. BookScouter will show you the prices offered by a variety of booksellers, along with user ratings, and you can choose which one to sell to.
Once upon a time, many cities and towns had bookstores that specialized in collecting and selling used books. Depending on where you live, there may still be shops that will either pay for used books, accept them for store credit, or just take them off your hands.