I am working on a startup in this space as well so naturally I'm very positive towards the peer to peer marketplace business model. We are choosing to focus on community and local bid, ask, barter and trade. It will be very socially integrated. We will provide some middle man support like payment options, escrow, and helping predict the value of used goods but other than that it will just be a glorified craigslist.
I'm also curious what other people think the value propositions and qualities must be included to compete with the alternatives of both Craigslist and ebay.
We are in Beta but only a small subset of individuals that live in south east idaho and we have been having very good success after just 2 weeks. Please PM me if you want a preview or would like to talk more.