Many towns have used book stores that are looking for books of all kinds. You may not get a lot of money for your books, but it is a quick and simple process to drop them off.
If you are trying to sell textbooks, try taking them to your college book stores. Usually at the beginning and end of the semester, companies are at the stores looking to buy your used textbooks for a small fraction of what you paid for them.
Amazon.com is one of the leading websites for resales. It is easy to register for an account and type in the name of the book. It will ask for brief information about the book, such as its condition, and then suggest a price. If you are not happy with the suggested price you can name your own.
Half.com is a sister website to Ebay. This website works just like Amazon.com, but gives you a broader range of customers which can be beneficial with more people viewing your product.
List the books you want to sell on Craigslist. You do not need to set up an account, and people will come to you if they are interested in purchasing the books.
Rare and old books can be sold at antique shops, and depending on the condition dealers will offer you a price. Collectors shop antique shops looking for original copies or rare editions of books.