I have an idea for the rating system.
When the buyer wants to leave a review they could split it into categories, for example, one for support, one for product functionality etc etc.
Functionality (3/5): The product worked great, but needed additional tweaks to make it work perfect with my wordpress installation.
Support (5/5): The support for this product has been excellent and punctual.
Overall (4/5): Could do with some minor tweaks, but decent product overall, and it’s a steal at $x
Also, in order to help encourage them to leave more feedback, we could work on a loyalty scheme, such as leave 10 reviews and get $5 free credit.