There is a lot of trust involved with this. I don't want to put a downer on your idea, I really think its great, but I believe there may become a market for trusted accounts. People who build up trust and loose control of their account, or sell their account to an unscrupulous trader to take advantage of people. De-centralised is the dream, but a trusted open source mediator that can hold both currencies and then release them when both have been received as planned can remove this necessity for trust.