Indirect exchange site. You put in what you are willing to give away e.g. a CD and what you are looking for e.g. a coffee mug. The site trawls all the 'swap' sites (like Craig's list) looking for intermediate swaps that will achieve the desired result e.g. you have a CD, Tom has a tennis ball and William has a coffee mug. William wants a tennis ball and Tom wants a CD. The program matches you all up so you give your CD to Tom, Tom gives his tennis ball to William and William gives his coffee mug to you. A 'mega-barter' system like this removes the need for a currency ...
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.
Exactly! I heard a story about a guy who started off with a pen and ended up with a truck. I don't even know if it's true but it is inspirational. Actually that could be an interesting system in its own right - I have a CD and I want a car - show me the swaps available to make it happen ... though I actually think the major limitations for such a system would be an intractably large search space.