I hear this over and over but where's the evidence that this is true on a significant scale? Sellers have been counterfeiting and selling knock-off garbage on Amazon since the 3rd-party marketplaces launched.
3P sellers are a NECESSITY for Amazon. They source and stock the hundreds of millions of long tail items that Amazon themselves can't/won't. For Amazon the benefits of 3P sellers far outweigh the costs.
Edit: Who's downvoting this and why?