They could go with the eBay model, where only those who are either the buyer or seller can leave feedback. Before eBay went to this system, you could not tell if good comments were made up by associates of seller/buyer or whether bad ones were just a crank causing trouble.
Feedback system flawed?
Have you ever had a terrible experience with a member with a huge positive feedback rating? Have you ever read a feedback that is a lonely negative among many positives? Do you usually discount that lone negative feedback as a person with a chip on their shoulder? Thoughts and comment are appreciated.