THIS site is just amazing. You enter an ISBN of a book that you would like to sell, and it gives you a list what online book buyers would like to buy your book as well as how much they will pay you for it. Most will even let you print a prepaid shipping label. You can have them mail you a check or deposit funds into paypal.
We picked up some books for free today, and the very first one I entered is worth $30! WooHoo.
BookScout.com is awesome! Check it out.