Be sure to check with Amazon.com. They buy books for credit on their site. They supply the shipping label. You just need to box up the books, put on the label and drop them in a mail box (Or at the post office if the package is too big to be placed directly in the mail box)

On Amazon, you can look up the value of the book you intend to sell so you have an idea of what you will get for the book before you send it in.

My daughter sold the books she didn't want on Amazon.