Not allowing reviews from people who never purchased the product would be a start. The next level would be no reviews until the product is actually delivered.

Not allowing reviews from people who never purchased the product would be a start. The next level would be no reviews until the product is actually delivered. One more level up, no reviews until a reasonable amount of time post-delivery. Yet one more level: Require contacting the seller with grievances before posting a review. I would also add a finite impulse response filter to reviews in order to control rate of change. Rapid negative review posting rates should red flag and rate-limit these review. Also, the star rating math has to change. If you get just a few negative star ratings it can absolutely sink your listing. Unless you have thousands of positive ratings the math is such that a handful of one star reviews will destroy you; requiring hundreds of 5-star reviews to counteract. The battles being fought behind the scenes on Amazon are beyond what anyone would believe. Someone can take out any competitor instantly by making a few fake quality reports to Amazon. It’s insane, really.