I added a new link on my page for a very cool website called Cash4Books. This company will buy your used books (in good condition) and pay both the shipping costs as well as a fee for the books. They specialize in mainly non-fiction, textbooks, etc. and have a very easy sign-up form. You enter a books ISBN (located near the barcode on the back of most books) and if they are currently buying the book(s) will immediately quote you a price. If you have books you no longer want or need I would highly recommend trying this website. Just click on the link on my sidebar and check it out for yourself!
(If you find you like and use the service you can post a link on your own website and earn money for referrals of your family, friends, and readers.)