You can sell you unwanted book using Cash 4 Books: Use bonus code "BLOGGER3", Sell a minimum of 3 books and get an extra $5 bucks on your order good of all of 2012.
There is also a free iPhone/Android application to scan bar codes. You are given a free pre-paid mailing label (you do not pay to ship books). If you sell $35 or more in books, you may use FedEx (which gets you paid faster). You are paid via PayPal or check.