Can you elaborate about counterfeit products being sold under "sold by amazon.com"? That's surprising to me, I treat that as a sign of something being non-counterfeit.
Amazon commingles[1] inventory. So if there are N vendors selling an item, including Amazon.com, all N inventories just get mixed together at the warehouses. So if some M of those N are counterfeit, there's no way to know.
"As an example, if I sell Duracell C batteries on Amazon through their “Shipping Fulfilled by Amazon” — which I must do to receive Prime shipping designation — I need to send my batteries to an Amazon warehouse. After receiving my delivery, they will count the number of batteries, then slide the whole stock into a generic shelf labeled “Duracell C Batteries.” Any purchaser receives a Duracell C battery from that box, and thus the actual seller is unknown."
I’ve ordered an Apple-brand Lightning cable from Amazon (sold by Amazon), for example, and received a counterfeit.
They replaced it, of course, but had it been a gift, or I’d been in a hurry, would anyone have noticed? If Amazon can’t keep counterfeits out of their own inventory, what chance do most buyers have?
My understanding is that all sellers, including Amazon itself have co-mingled inventory. Therefore you can’t actually guarantee that what you’re getting is from Amazon’s stock, as opposed to some other random seller who gave the FBA warehouse a truck full of fake products.