The deal combines the huge inventory of cars listed with AutoTrader with eBay’s millions of customers who bid for a wide variety of items online. The companies said their venture will become the biggest player in the emerging market for online car auctions.

eBay, which already operates a relatively small online car business, said it would make a cash investment in AutoTrader to “solidify” the deal, but did not disclose its size. AutoTrader, which has print and online units, is viewed as a candidate for going public sometime during the year.