Sell Used Books Online
This company also buys directly from you.
You would enter your ISBN number on their site to get a quote; they also pay for the cost of shipping the book out to them.
9. Powell's Books
They allow you to type in the ISBN number of your book to see if they would even accept them, and then on top of that, the book would have to be in an almost perfect (like new) condition.
This site also requests that you enter an ISBN number and, if they want your book, they will purchase it directly from you and send you a pre-paid shipping label.