There are tons of websites out there that let you sell your used books. But it can be a tedious process trying to figure out which one will give you the best prices. Instead of going from site to site, you can simply enter the ISBN number on BookScouter.com and it will search all of them for you. I've tried this site and checked the results on the individual sites and all the results were accurate.
The results also tell what methods that site will use to pay you and customer ratings. They even have a new mobile app!
I found BookScouter to be a highly useful tool and I highly recommend it!