EBay is tapping Encino-based Bid4Spots in a new service that allows advertisers to bid for radio advertisements. The new Ebay service was expected to launch today as part of eBay's Media Marketplace. Encino-based Bid4Spots operates an online weekly auction of radio airtime, which is a reverse auction of unsold advertising space. The launch of the service puts Ebay head to head with Google in taking on radio advertising, which operates a version of the firm's popular AdSense advertising program for the radio market.