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My application for Ebay got rejected. Don't know why. Amazon affiliates approved easily. And they have good support in terms of product banners, links and setting up your own Astore.
People automatically associate Amazon with having the book they need (I've never come across a situation where Amazon hasn't had a book I'm looking for) so there is more of a chance people will search for your book there.
For everything else, I would say eBay. BUT I would recommend you wait until they have a free listing weekend. That way, you save on the cost of listing your items because the listing and selling fee can take a decent portion of your profits away.