It is perfectly realistic that that stay out of businesses they can't competently run.
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.
