Hi Jonathan,
Letting users edit reviews/ratings, as well as the ability for the producer to reply, is imperative here. However, having the option to personally contact the customer via the system would also be very helpful. This is an option in the other 3 sites that I currently sell through. In the handful of cases where I have been left a bad review, for any reason, I have been able to satisfy the customer to the point where they changed their review to something positive. There was only one exception, and even she changed her review to 3 stars instead of 1. So, out of more than 3,000 sales on Etsy, for example, I have a solid 5 star average.