Pay professionals to pick and vet reputable, high-quality vendors.
Costco is actually a very bad actor in this particular area; they generally order their stuff with a unique SKU (often the same product, but just a different SKU) specifically so that reviews of outside products don't apply to the (again, identical) Costco product.
More product categories than Costco. Fewer options within those categories than Amazon. I can't buy an Apple Watch screen protector at Costco; at Amazon I'm presented with 8,000 Alibaba resellers, often with identical photos of the same product.
For example:
https://www.amazon.com/Lalagen-Women-Sleeve-Peplum-Blouse/dp...
https://www.amazon.com/KaleaBoutique-Womens-Sleeve-Cutout-Pe...
Two different "brands" (there are probably dozens of other sellers with the same item, but these two show up on the same page) selling the exact same shirt, with the exact same photo. I want a site that doesn't allow this, by sourcing reputable vendors.
The problem isn't fake reviews per se, the problem is products not matching the reviews. You can check a sample of products then stop people who are responsible for "fake" products from being involved in Amazon selling in any way.
Basically do "Which" style consumer reviews on products being sold.
Trusted reviews are better left to 3rd parties.
I guess Amazon have no reason to implement any sort of techinical checking unless it gains them customers, so people have to vote with their money first.