As far as I can see a lot of buyers don't rate the scripts, simply because they don't know they can rate it, or they don't know how to do it. My suggestion is to let buyers to rate the scripts purchased directly from the Item's page, I know it's already simple to rate them from the Downloads section but a lot of them don't know this, or at least an encouragement message when they visit that page.
I have an item (I'm not gonna name it) which was purchased 50 times, but was rated only 3 times, basically only 6% of the buyers rated it, and today someone had a little trouble with the respective script, I've helped him and asked nicely to rate it, his message was exactly the following:
Works GREAT. Where do I rate this product?
I'm pretty sure you (the authors) have encounter such issues too, and this 'minor' tweak would help the marketplace in my opinion, what do you think?