I believe that Facebook should work on Ebay 2.0, the social graph evolution of the traditional auction market.
On Ebay, a user can rank another user according to the quality of the transaction they concluded. Usually, each user take a look to the seller's ranking for assessing the potential risk of a bet. Even if each user is identified with a login and a password, the seller and the bidder do not know each others.
Facebook and the social graph will add an additional level of trust since both the seller and the bidder can identify a path of users that connect both of them and can provide references. Linkedin adopted a similar mechanism for introducing each others unknown persons. Facebook can use it for creating a new Ebay. In alternative, they can buy a competitor and integrate it in the social graph.