eBay is auctioning unsold inventory from 2,300 US radio stations. Advertisers can buy airtime using the eBay Media Marketplace which was originally set up for cable tv ad auctions. The service effectively rebrands the Bid4Spots efforts to open a radio media space auction marketplace. It’s a reverse auction system where advertisers declare how much they will pay for airtime and radio stations compete by making offers to win the business. Bid4Spots and eBay are partners in the operation.