According to Reuters:

“eBay is set to unveil a keyword advertising system for eBay sellers to promote auctions on other Web sites, borrowing from the strategies of Google and Yahoo.“

The system is called eBay AdContext, and like similar Google and Yahoo systems, Ebay will analyze the content of a web page and deliver relevant auctions. The advertiser will receive a cut of the proceeds, which will be between 35 and 60%, depending on the volume.

Screenshots here.