Make money on those old books

So I visited www.cash4books.net, they buy old books. Might as well make some money on them. All you need is the book's ISBN. I tried several books but out of about 5, they were only buying one. They offered me $2.49 for it (I think it was Dog Training for Dummies). I decided to keep the book but I know where to go if I need some cash for my books. As for the books they are not buying right now, I recommend either holding onto them and trying again at another time or donating them to your local library.