eBay has launched another foray into the ad market – this time as a broker of airtime on over 2,000 radio stations in the US.

The move expands on an existing venture by the auction giant into cable TV ad sales.

According to Reuters, eBay has partnered with online radio ad marketplace Bid4Spots to power the new service, which will be called the eBay Media Marketplace for Radio.

It will go live later today, offering ad space on both terrestrial and online radio stations. It will also include inventory from Clear Channel, the largest radio player in the US, which is already selling ads through Google’s own radio ad exchange.

eBay has long been seen as a potential rival to Google et al in online ad sales – especially with growing use of online exchanges for a range of ad types.

Speaking to us recently, Mike Walrath of Right Media, the Yahoo!-owned display ad market said it was “logical” that eBay would be interested in its space. The auction giant sees its existing base of sellers as potential advertisers in the online ad markets it develops.

Predictably though, it has experienced resistance from some quarters to its existing cable TV venture.