For most of history people selling things had an understanding of what they were selling. If Amazon can't meet that standard for some products, or for third-party sellers as whole, they can just stop.
Alternatively, they could set up a separate site. Something like "Caveat Emptor" or "Bezos's Dubious Flea Market". Something that doesn't trade on the Amazon brand and create confusion between responsible vendors and fly-by-night operations.
That you can't or won't think of a way to do something without harming megacorp short-term profits doesn't make it "unrealistic" to address a recently create problem. I leave it to you to figure out what it does mean.
It is not my problem to solve, it is Amazon's. If they can't solve it then they need to go back to selling their own stock. If this makes them less money than facilitating a crime, then so be it.
None of this is realistic. We're commenting on an article that says Amazon has half of the e-commerce biz right now. The small but vocal contingent on HN might seem to make it that counterfeit goods are rampant everywhere but it's not the case.