It was pretty obvious what Amazon wanted from its business partners if you look at abebooks, a site owned by amazon, but not visibly if you just look at abebooks.com. 'Independent' booksellers sell used books through abebooks, but never for less than amazon sells the same books.
I never realized AbeBooks was owned by Amazon, but I've never bought a book from there because the Amazon price was always the same or better. Now it makes sense why.
I do not have a reference right now, but I remember reading that one of the conditions for listing your product on Amazon is that you cannot sell it for less anywhere else.