I applaud ideas for distributed exchanges, so I play devils advocate here in a constructive way. The first concern I have here is potential legal problems for users. Say I deposit 1000 dollars in the account of a drug dealer (of course I had no idea of this) that gets catched afterwards by the police and his bank account is investigated. You bet I will have problems.
If a person gets caught for selling drugs and I bought socks or soda from him that does not criminalize me. How would it criminalize me if I bought digital bitcoin goods from him? I am not part of his drug trading activities and I can not police all the activities of every stranger that I conduct business with. I am related to the stranger solely for what my business was with him - it would be buying or selling digital goods that I have acquired or would dispose of by buying or selling it or keeping it. You can not be held accountable for any criminal deeds of people you conducted business with if you were not involved in the criminal deed! If I sell someone fresh fish at a market and he keeps it unrefrigerated for days, and serves it to a customer at his restaurant that acquires food poisoning in the process, the negligent criminal act would be due to his actions, not my due diligent actions.