Lawsuits. William Sonoma and Mercedes Benz have both sued Amazon claiming they sold counterfeit. Popsockets recently told Congress that Amazon had sold counterfeits (although as I commented in the recent HN thread on that (see
https://news.ycombinator.com/item?id=22079904), popsockets has been bullying other sellers in ways I view as illegitimate, so I take anything they say with a grain of salt.)
This is nothing unique to Amazon, by the way. I could show you lawsuits against Walmart and Costco for selling counterfeits. Every massive retail company that doesn't buy 100% from the manufacturer is going to eventually have some leaks. You buy from the grey market, vast majority of the time you get legitimate cheaper product, occasionally someone scams you.
Oh, and Apple famously sued Mobile Star LLC claiming that they had sold Amazon counterfeit products through Vendor, which was "sold and shipped by Amazon.com".
Links to all above cases:
https://www.courtlistener.com/docket/6173730/daimler-ag-v-am...
https://www.courtlistener.com/docket/4495986/apple-inc-v-mob...
https://www.courtlistener.com/docket/8418854/williams-sonoma...