All you have to do is click on the link above or the image below and it will take you to the main page of their website. When you get there, you will see a box on the left hand side that says enter your ISBNs. You can either enter them all at once (make sure you hit enter after each ISBN to start on the next row) or you can check them one at a time. Once you have entered the information, click on price my books. Then you will be taken to another page that will show you if they will purchase the book and how much they will give you for it. All you have to do is follow the step by step instructions, print a few pieces of paper (a packing list & barcode). If you have five or more books, you can send them via FedEx. If you have less than five books, you have to send them via regular mail. You can receive your payment by having a check mailed or you can choose to be paid through Paypal. They do offer a 3% bonus if you go the Paypal route. Voila! You've turned books into cash.
I have sent a few boxes and this site is legit. They are a BBB accredited business and I haven't had any issues with them whatsoever.
Oops, almost forgot to tell you something important. Just because they don't accept one of your books today, doesn't mean that they won't buy them in the future. I've had a few that one day they didn't want and the next day they bought it.