To be honest, these concepts were made multiple times before (BitMit, Coingig, Cryptothrift) but few succeeded because of the complexity of such system to run stable on 15000+ users.
I was learning about Bitcoin and came up with this idea myself, and then did research and I indeed concluded there were multiple attempts and sites that have this concept.
No idea was stolen, and everyone is free to work out ideas? Idea's can't be patented or trademarked.
Yup, people keep trying to take a stab at this one over and over, no one quite has pulled it off yet. I think Openbazaar is probably closer at building that "P2P" decentralized marketplace that everyone has been hoping for. It's classic chicken and egg though, which is partly why the others you mentioned failed. eBay has spent a fortune building a customer base and a sellers marketplace, like Walmart and Costco in the brick world, no one likes it, but we all shop there even though there are alternatives. I think one day one of these (or yours) may succeed, but it's going to be a very steep hill to climb over a long, long, long time. Best of luck to your project though.