Blowfish has been responsible for elements of eBay's online direct response and brand building activity since its appointment in 2006.
The extra work for Blowfish will involve it promoting using effective keyword buying and creative management across the larger search engines. This will complement eBay's in-house online marketing activity.
Minal Shah, head of internet marketing at eBay.co.uk, said: "Blowfish Digital's creativity in developing new media partners has helped maintain our online brand prominence in the UK and they have become an integral part of the team here."
Blowfish was established in 2005 and has a client list including Agent Provocateur, La Senza and Jackpot Joy.