I have gotten a lot of books via alibris.com (no affiliation, just a happy customer), a marketplace for used booksellers. I got a brand new $170 statistics textbook for about $35 because it was intended for another country; thinner paper and crappier binding, but the same book. I've also gotten used books that were indistinguishable from new books.
Not as cheap as ebay, but an enormous selection with pretty reliable quality ratings.