I'm thinking about developing a market software that supports NashX transactions (a mutually assured destruction equilibrium strategy). If properly implemented it should be scam proof, since you won't have to trust neither the market admins nor the seller. At the worst possible scenario you'd lose one transaction's worth of money, and sellers would have nothing to gain by scamming buyers.
Is this a good idea? Is there anything similar already out there?
I think you know this is a good idea. But the implementation and execution are the difficult parts.
The only thing similar to this would be multi-signature escrow that has been implemented by TMPSchultz and his crew over at The Marketplace on the i2p network.
Regardless, I'm sure a lot of people would be interested in trying out your market once you have a workable version running on an .onion/.i2p site.