Wouldn't that require the escrow agent (your facilitator) to either only make deals between active users (so that they can sign the transfer transaction from Alice [selling] to Betty [buying]) or for the escrow to have some multipart key with Alice that has a blank recipient address? I'm not sure we've gotten away from the escrow/exchange storing BTC there and it sounds like a very unliquid marketplace.
Yes, it would require deals to happen between active users. This does to some extent reduce liquidity, but I am skeptical that such liquidity is more valuable than security from having all your money stolen.
