This is going to be a huge problem in the medical/health vertical. It's one thing to get a BommyTahama pair of sunglasses, it's quite another to get counterfeit supplements that could cause injury.
(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.
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...
Best avoided imo.
They simply cannot be trusted for these kinds of things.