Cash for books

So, here is another thing I just discovered if you are a voracious reader like me. In reading my free Woman's Day magazine recently, I saw a little blurb about a Web site called Cash4Books.net that will PAY you for your used books. I thought, wow, I need to check that out. So, a book I recently read I did a search on the Web site for how much they would pay me? $4.38, not a ton, but considering I got this book for free and probably won't read again, I thought, hey that's worth it. I did a few other random searches on books around, but they weren't accepting those, so it may depend on what they have in stock if they want your book.
What's the catch? I don't see any so far (I haven't sent any books yet), but they pay the shipping to have you send your books, so this is an easy win. Now, when I have some time, I'm going to review all my books on my bookshelf and rather than just donating them to DI or somewhere, I'm going to make some money off them. Not that donating them to your public library or DI isn't bad, but hey, FREE MONEY!