[junto]

The challenge is in the actual exchange of the physical goods. You make the effort and take the time to meet Tom in a coffee shop downtown in your lunch break and hand him the CD after he turns up late. Tom is a lazy ass and doesn't bother to meet William and ignores his pleading emails. You are then down one CD and one coffee mug. There is a good reason why money exists as a primary form of exchange rather than barter.

I had a similar idea for an app, but kept hitting physical barriers to implementation.