The groups fear that sellers are innocently trading items on the web, unaware that finds should be reported under the provisions of the Treasure Act before they can be sold.
Through the arrangement, a team set up by the museum's Portable Antiquities Scheme will check auctions on eBay and report illegal sales to the Metropolitan Police's Art and Antiques Unit.
The two organisations have also produced a guide for sellers which they can check to make sure they are not breaking any laws.
Dr Roger Bland, head of Portable Antiquities and Treasure at the British Museum said: "Our experience is that most people who buy and sell UK archaeological finds do so without being aware that they may be breaking the law if items have not been reported.
"We have therefore worked with eBay to ensure that its users are aware of its obligations through our guide. We will also be contacting sellers to ensure that they have reported items and have appropriate documentation."