abebooks.com links to a lot of used booksellers. betterworld.com is my favorite that I've found through there - not always the lowest prices, but usually cheap, and free shipping which is a huge plus with something as heavy as textbooks. And they are supposedly socially responsible. My only problem with them is that the shipping can be slow - don't wait until the last minute to order! But I expect it's no worse than most online booksellers shipping via media mail.
(I hope I'm not making a mistake advertising them before buying my own books!

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