What if, instead of focusing on products (which might have been mercata's downfall?), we focus on pooling money for the local community? It's a fundraiser service, except the service will also take care of several loose ends.
(1) people bid real money to get something done in their local community (fix potholes, install solar panels, bring music bands into their local bar, whatever).
(2) the service builds web tools to help the community discuss options and reach a consensus about a solution
(3) people can opt out at any time until the proposed solution (reached via community vote) gets implemented (by contracting out or whatever). This gives the users the power of choice.
($) the service primarily makes money when fundraising communities can reach a consensus. There are also infinite possibilities for making deals with meat world service providers (construction contractors etc) at a large scale. Also, the money that users pledge will actually be deducted from their accounts and held, accruing interest, until the user opts out or a solution is reached.
But the key to their success has been their customer / market validation, and the singleminded pursuit of their target demographic (which is itself interesting, as they're targeting 25-40 y.o. females in a high socioeconomic class).