There is nothing that can show you how good model is without buying it and checking it by yourself. Something that is good for you doesn't neseceraly mean that it will be good for me or someone else. Number of sales of particular model doesn't show anything, because in most cases (I personally never heard that someone recommended 3d model to someone else) customers doesn't recommend models to each other and rarely communicate. Reviews of model can be used for deteremination how good model is but but only if artist is honest and legit, but even with that you can't be 100% sure. There are peoples who will write the baddest posible review in order to justfied their refund request, because they want to get model for free (thieves) and that one review (if there isn't positive ones) will trick customer to think that model is bad. If requested, support will sometimes clear that review and decline refund, sometimes is the opposite, it depends on a situation. As far as positive ratings goes, you can be sure with those either, because there is a new "very modern, very smart, made by very intelligent and smart people" trend of creating multiple accounts and buying model from yourself in order to give yourself 5 star ratings wich will trick customer to think that your model is good. Don't be surprised when you see a very bad 3d model which price is 2$ , to have 3-4 5 star ratings, in 1-2 hours after upload, the model that I personally wouldn't download even if it's free.