What should I do with my obsolete textbooks?

Half-price books is the ONLY place that I have found to buy my obsolete textbooks. It really is amazing, huh? I'm a sopre in college now, and it's like there is a new edition of everything almost every damn year- they pile up quickly. You won't get a lot of money from half-price books, and I don't even know what they do with them; but at least they will be gone, and you get a little something back.