It's an interesting idea.. but it seems like it's centralised on a single website, which is a single point of failure (and vulnerable to things like DMCA takedown notice).

It may be that you can't solve the problems without centralising, but I'd hope someone smart would come along and come up with something better.

As far as chargebacks go.. you'll end up with a messy dispute mechanism if you're not careful.  In the case of fraud they're invaluable (and you'll never be able to safely take credit card/paypal without such a mechanism, and the dwolla mess wouldn't have been an issue with it in place) but user A saying user B didn't send an item through the post?  That's how paypal came unstuck...