Everyone is talking about the eBay & Yahoo partnership, where Yahoo will be eBay's exclusive provider of graphical ads and Yahoo will promote eBay's PayPal to its merchants and publishers. Reports via the Washington Post, The Street and BusinessWeek.com stress how this poses a threat to Google and Microsoft. It is important to note that this partnership is primarily to provide graphical ads and click-to-call ads and on a lesser standpoint to provide some search ads. The limited search ads are probably because eBay does not want to detract visitors from the eBay products and auctions, which is logical.
Postscript: Hitwise has some stats on traffic that flows from eBay to other sources.