The win coincides with the arrival at Seventy Seven of Matt Wood as managing director from eBay's previous agency Joe Public Relations, which declined to repitch for the account.
Wood said he had had no influence on the pitch result, explaining: "Seventy Seven was already on the pitch list when I joined and contractually I can not work on the business for the first six months."
Board director Guy Corbett, who leads the agency's seven-strong team on the account, said eBay needed to proactively communicate the breadth of its services.
Seventy Seven's brief includes promoting PayPal, eBay's secure payment system, and responding to media reports of fraudulent deals through the site.
The pitch followed the appointment of eBay's first UK head of PR Victoria Sayers earlier this year.
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