Maybe just a simple request (in automatic generated e-mail ) for leaving some comment or ranking feedback about the products purchased on CGtrader would already make big difference?
Preferably this automatic e-mail goo's out 2 to 3 days after purchase to leave some evaluation time.
This e-mail would then contain images of the products purchased and question "how would you evaluate this products purchased 3 days ago on our community of designers" (then give scale from 1 to 10 to rank).
This would be much like the community ranking game but then for the clients own specific purchases.
Maybe, also little fields to leave some comment linking directly to the specific products could be added in it?