It's not that hard - look at systems like Fakespot that could automatically verify reviewer credibility (without even access to the internal data that Amazon holds).
Or just offer systems for buyers to report suppliers doing exactly this - buying reviews.
Or honeypot it, and just put some fake addresses on sites that sell reviews and then ban companies that send you stuff.
I mean there are loads of solutions, but all of them involve Amazon taking down 5 star reviews that help them sell crap, so the incentives are obviously conflicting here.
Re:Amazon Review Scam
by LorenPechtel on Tuesday November 10, 2020 @02:31PM
(#25050538)
He's talking about getting your competition banned by false-flagging them. It's much harder to figure out who is placing the fake reviews than that they are fake.
It's a hard problem. The solutions most likely to work are also expensive. Amazon could create internal product review teams staffed by employees instead of letting users do it all for free, but it's hard to make that scale. It's also less transparent; one of the big appeals of Amazon early on was the novelty that you could go to a retailer and if you don't like one of their products you can say so for the world to see. Now most other retailers do the same thing and we sort of take it for granted now.