If you have some old textbooks you don't need anymore, there are two great sites I've found that you can sell them to.
The first one is TextbooksRUs.com. You go to the site, type in the ISBN number of your books and the site will tell you right away what they will give you for them. I sold some books to them for $36.97. They also pay for shipping.
The second site is CollegeBooksDirect.com. This site works the same way as the other one. You just type in your ISBN numbers. They also pay for shipping. I sold some books to them for $12.
Those amounts might not seem like much, but the books I sold to them were actually very old (for textbooks, anyway). Most of them were from 2002-2003. So the fact that I was able to get any money for them at all is pretty surprising. So if you have newer books that you want to get rid of you could probably get a pretty good deal for them.
There is one site that I absolutely DO NOT RECOMMEND and that is Cash4Books.net. I sent them some books and they were supposed to pay me $22.38. But they sent me an e-mail stating that they would not pay for the books because they smelled like cigarette smoke. I find it fascinating that the other sites purchased my books even though they were stored in the same location as the books I sent Cash4Books. But what I really find amazing is that my books supposedly smelled like cigarette smoke even though I don't smoke! They did say that if I wanted my books sent back that I should e-mail them within 30 days, but I didn't bother. The only reason I sent them to Cash4Books is because the other two sites weren't buying those titles. I'm pretty sure Cash4Books would have wanted me to pay for the shipping too, so it would have been a waste of money because I couldn't sell those books to anyone else, so I would have had to dispose of them anyway.
They also said that if I didn't want my books returned to me that they would recycle them. Somehow I can't help but wonder if they really recycled them or if they sold them without paying me for them. Either way, sending my books to them was a complete waste of time. I can't exactly say they're running a scam, but I do not recommend doing business with them.