There's a point to be made about how it's not possible for Amazon to check every single listing (made by other companies) for authenticity, nor they can use user feedback as a way to check/block those companies as many people use a different return reason than the real one (eg. "I don't like it" becomes "it's counterfeit").

It's unfortunate but you'll have to learn how to navigate the new amazon, and possibly use the mentality that everything CAN be counterfeit on this platform.