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.
Tech books for the Indian market have been a good resource. The books (and maybe exercises) slightly differ to the US prints to keep control of the market.
I acquired a huge stack from an Amazon UK seller for literally pennies each.
The academic text books are known to have different test questions so US students can't buy (relatively) dirt cheap "foreign" prints. The text might be the same elsewhere in the book though.
Not as cheap as ebay, but an enormous selection with pretty reliable quality ratings.