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http://www.quora.com/How-many-registered-users-does-eBay-have
https://investors.ebayinc.com/financial_history.cfm
https://www.google.com/?gws_rd=ssl#q=eBay+Metrics.pdf
According to eBay's Unaudited Supplemental Operational Metrics (released January 16th, 2013) the company has 112.3 million *active users.
*Active users are defined as any user who bid on, brought, or listed an item on any eBay Marketplaces trading platform, excluding Half.com, Stubhub, and any of their Korean Subsidiaries, during the proceeding 12-month period. Users may register more than once and as a result may have more than one account.