In the grocery store, in the future

You go to the grocery store but you don't have any money so you wander over to the Internet kiosk and pick up a slip (from Craigslist?) that says, "Bring me my groceries and I'll pay you five dollars." You pick up a bunch of these slips and shopping for your neighbors pays for your groceries plus gas. Maybe the people for whom you're delivering don't have cars or maybe they're just busy.

It's sort of like Webvan but informal, unstructured, and cheap. Why aren't we doing this already? Maybe we need pre-written grocery slips.