As someone who purchases things from Amazon and only care about if they work or not, I haven't had much an issue. If I've been loading up on counterfeits, it doesn't seem to have been a problem.
I emailed Bose. They asked for the serial number and photos. They said "Yep, it's fake." That was all. They didn't even want to know who sold it to me. Amazon support also did not seem to care. They just told me to send it back.
Amazon does have a brand registry which manufacturers can opt into and then actual proactive counterfeit enforcement will happen specifically for that brand. Some but not all items in the program have unique per-unit barcodes that Amazon verifies and customers can too through an app called Transparency. But as a customer you have no way to tell which brands are enrolled in the program unless you receive the item and see the Transparency barcode on it.
Yes, because Amazon co-mingles item of the same SKU. A legit item could be shipped to someone who buys from a scammer, and a counterfeit could be shipped to someone who buys from a legit seller.
* Seller A creates Listing Q with Sku U (real)
* Seller B creates Listing W with Sku U (counterfeit)
* Amazon organizes stock by Sku
* Ordering from Listing Q or W will pay to the appropriate Seller A or B, but get a random product with Sku U