Microcommerce

What do I need to do in order to get a little microcommerce around here?

I have a very real problem: buying groceries in small quantities is
tough. I want to be able to buy and sell extra groceries, but I want
to limit transactions within the building. (I don’t want to have to
walk out!) Some things I’d be willing to buy and sell further off, I
suppose, but I’d love to have classifieds just for Graduate House.

So I need classifieds that are easy to monitor. Special considerations
for groceries would also be useful (example: expiration date, being
able to easily track how many units are left). I want RSS feeds with
fancy criteria. =) We can handle payment offline, so that isn’t really
a concern.

Hmmm. Sounds like a fun thing to build over Christmas. Plus points if
it hooks into Mambo, I suppose.