Amazon gave people and companies the ability to sell on in 2000, and it has since grown into a juggernaut, representing 49% of the goods Amazon ships. Amazon doesn't break out numbers for the portion of its business driven by independent sellers, but that translates to tens of billions in revenue a year. Out of more than 2 million registered sellers, 100,000 each sold more than $100,000 in goods in the past year, Peter Faricy, Amazon's vice president in charge of the division that includes outside sellers, said at a conference last week.

It's clear, after talking to sellers and the software companies that empower them, that the biggest of these vendors are growing into sophisticated retailers in their own right. The top few hundred use pricing algorithms to battle with one another for the coveted "Buy Box," which designates the default seller of an item. It's the Amazon equivalent of a No. 1 ranking on Google search, and a tremendous driver of sales.

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