For years the best place I’ve used for used books (especially those out-of-print or rare books) is AbeBooks.com. I just set up an affiliate account with them, so any purchases made through my blog end up supporting this site.
Frankly, I wish this had been around when I was an undergrad mainly for the purpose of buying and selling textbooks. (You can buy new books there, too.)
Shipping is free, there’s a 30-day money-back guarantee, the average saving is 50% off, and you can sell the textbooks back to AbeBooks.