Amazon says most of its more controversial works end up on its site through the Amazon Advantage program, which lets small independent publishers sell their wares on the site.

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But it does not give Amazon any advantage over other on-line retailers, who by also using Ingram immediately have access to a similar range of books.

ECONOMIST: On-line retailing

He adds that his study finds that if the Amazon sales tax advantage goes away, Amazon Prime members would pay a modest premium to Wal-Mart store prices and about the same prices as Target customers.

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