The hardest part is to build enough momentum to have a viable marketplace otherwise you're stuck with a chicken and egg problem where no one posts because no one's buying and no one's buying because no one's posting.
I have a novel idea that I'm happy to share (and work on if you're interested) and that's a classifieds marketplace with REAL identities. I've sold and bought many things on Craigslist and Ebay over the years and I've come to value and appreciate people who don't give me the runaround or try to scam me. I think a lot of the good folks who I do meet when I buy or sell feel the same way about anonymous online classifieds. I personally always feel uncomfortable inviting complete strangers to my house to demo an item I'm selling w/o knowing a bit about who they are and if they're a known scam artist.
Some sites like Replyboard are trying to solve this with reputation and ratings for classifieds sellers but to a large degree they are hidden behind screen names and such. I would love to buy and sell locally to REAL people who are willing to back the stuff they sell with their name/identity. How would I do this? I would require two concurrent identity logins > Facebook & LinkedIn. I know this will limit the number of people who would participate but you would be surprised at how many "professionals" fit into this group.
Let me know what you think and best of luck with Classfy!