You're allowed to provide feedback on the service, just do so in a respectful manner. Every experience isn't necessarily positive so every review isn't either (ex: yelp.)
There really should be some sort of feedback system that is immediately visible. Odds are no one would see even if he did post. It would get buried quick.
Feedback rating systems are easily abused. People sometimes just make temp accounts to boost their positive ratings so they can run a scam. There is also problems of people leaving random negative ratings against competitors on throw-away accounts. This happens so often on the sites that have it. In short, I don't think it will reduce the frequency of scammers.