Just an idea/suggestion 
Reviews are really helpful to read when deciding what field/fields to buy next, but there are not so many people that write them.
So if possible, to encourage more people to write reviews, maybe each review you write you could get a 5 or 10 % discount for your next purchase, and to motivate people to write more than one review (because I think many people buy more than one field per time) perhaps you could add let’s say 3 reviews × 5% and get one 15% coupon (max amount). Not sure if there is one time use coupons, but that would be good.
And to qualify, one would have to post a review on both gumroad and the forum, using the same forum name on gumroad so it’s easier to verify. And the review would have to be put some effort into, not just “good field 
”.