I think facebook should try a groupon style model. They have locations and intentions of users. They should just have a deal feed, or some type of email listing with possible low prices they could get if they join in a deal. To filter it better, maybe ask each user for a list of things they want cheap.
Facebook has tried copying Groupon. This is their Groupon Now! clone announcement (http://www.facebook.com/blog/blog.php?post=446183422130). www.facebook.com/deals appears dead so not sure if they've abandoned the project entirely.
