Clearly Amazon cares. Not enough to fix the problem, but enough to build a new brand of white-label goods. If anything, untrusted reviews is to the benefit of the AmazonBasics brand.
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Amazon Reviews are a benefit to the 3rd party marketplace, more so than anything else. Buying non-branded goods from unknown (or nearly unknown) brands. I'm not entirely sure to see how, or why, untrusted Amazon reviews is Amazons' problem.
Even books now are often something that looks like it was printed on photocopy paper from a word processor.
I think amazon is converging to its niche (maybe not a niche if there is ~a trillion in annual revenue at stake) as a Walmart replacement. For instances where quality matters, I think they are already off the table for many people.
We'd really like to align the incentives of the reviewers with potential buyers. One way to achieve that would be to pay reviewers as judged by the people who buy the product from the service. There are a few problems here, like there not being much incentive to review low volume products, or manufacturers introducing 10k variations that differ by a single letter in a GUID (monitors/tvs), or a change in the quality of the product itself. These sound tractableish.
Outstanding questions: does the math add up such that the incentive of maintaining a high quality platform is larger than the incentive for manufacturers to defect (signal high quality and cheat). Can we pass on that win to reviewers?
Other thoughts: we can't punish manufacturers for cheating because its too easy for their competitors to abuse.