When you go to a listing (SKU) on Amazon, it shows you a bunch of "sellers" with various prices. These sellers usually fulfill with Amazon (hence, Prime shipping). Each seller sends its merchandise to Amazon's warehouse for fulfillment (shipping to you). Amazon commingles the inventory; all of that SKU goes into one bin, no matter who provided it.
Suppose the sellers are Creator, Honest Retailer #1, Honest Retailer #2, and Sketchy Knockoff. You care about quality, so you pay extra to order from one of the three trustworthy sellers. You still have a 1/4 chance of getting one of the Sketchy Knockoff's counterfeits, because they're all picked from the same bucket.
This also explains why you might see a review stating that someone received a counterfeit, even though they ordered from the creator.
(Yes, I have filed detailed reports with the FDA and FTC)
I've got counterfeit electrical goods on Amazon (solid state relay that was rated at something like 40A, but on opening it up after realizing it was counterfeit, found the electronics were only rated to something like 10A).
I complained in a review, that review never got published, despite the only SKU having only one seller - the purveyor of the counterfeit items. I did notice they blurred the logo of the real brand in some pictures later, though.
Best avoided imo.
They simply cannot be trusted for these kinds of things.
I think there have been a few different noteworthy events where issues have popped up. I'll link to a couple of articles I've found on the topics.
https://themedicinemaker.com/business-regulation/amazons-fda...
https://www.sitejabber.com/resources/counterfeit-cosmetics-t...