eBay ( EBAY) launched a service to let advertisers purchase spots to place radio ads. The auction giant said Wednesday that it will partner with start-up Bid4Spots to push into the $20 billion radio ad market. The move will also increase eBay's rivalry with search giant Google ( GOOG), which entered the market for radio ads in the beginning of 2006. eBay will allow advertisers to bid on spots on more than 2,300 radio stations. Most of the inventory will be made up of slots that could not be filled, since stations get higher prices for their prime-time slots than an auction might get them. eBay's radio ad business will be part of the broader eBay Media Marketplace, which also sells ad for cable TV slots. Shares of eBay were recently off 0.5% to $31.53.