I just have to wonder ... money has to change hands. Isn't there a cost to doing this, large enough to make it somewhat prohibitive? Even for really cheap items?
It could be that the seller is simply paying some shady firm to improve their seller reputation. The shady salesman tells the store owner "Give me X dollars and I'll make sure you have a 99% 'good' rating". They take them money, buy a bunch of cheap junk and leave the fake reviews. The store owner may not even know they are fake.
You would think so, but its really not because its a % of goods sale price. FBA merch fee is only 15%. That matters when items are cheap.
So for the cost of COGS + $5 product price *.15% + parcel cost ($4, lower for CN goods dropshipped from abroad). Assuming the good is made for pennies in CN, you are then talking less than $10 for a 5 star review.
Consider CPA in google , FB, or Amazon ("ACOS"), and you start seeing a trend. Keep in mind reviews scale, i.e. adword dollars are spent. Reviews accumulate. They are an asset, not an expense