Amazon was accountable for 60 percent of the online sales growth in the United States last year, according to Forrester Research. Much of those sales came from third-party marketplace sellers, which include many traditional retailers. Forrester estimates this revenue at 20 percent of Amazon's overall sales.

Amazon added more than 30,000 sellers and 300,000 businesses to its business marketplace platform in the past year and continues to add "thousands of customers" every week, according to Prentis Wilson, Amazon vice president.

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