The appointment sees incumbent Unanimis lose its prized account, which it has held for the past six years.
Ad2One will create a team of 12 people to work exclusively on the account, focused on offering advertisers innovative ways to target specific user groups on eBay.
"EBay's success is based on its loyal users and their strong sense of community spirit - whether they are buying or selling cars, cameras or comics," said Julian Miall, joint managing director of Ad2One.
"It is our intention to offer agencies more targeted and higher value advertising solutions that give them the opportunity to reach specific segments of the eBay community."
The two-year contract is effective from 8 April 2008.
EBay was ranked as the third largest web property by comScore in February, with over 21m unique users.