How often do buyers leave a review or simply good-bad

I agree a lot with the comment above, but also has to see a lot that the model you bought really serve them for what they bought, if so and the model is at a very good price according to their criteria, they will definitely leave you a good score.

I would say, less than 10% . At least that's my case. But it was even worse a couple years ago.
Some observations: people who contacts you for a model support and gets your help, tends to leave rating more often, people who contacts you with a price offer, very rarely rates purchased model, even if asked specifically.