But for many things I shop for, that would be extremely inconvenient for me. For example, I needed to buy replacement fuses for a Fluke multimeter. Amazon itself didn't sell it but a 3rd-party marketplace seller did. If the marketplace was shut down, I'd have to go through the hassle of registering a new account on yet another ecommerce website just to buy that fuse. In this day and age of customer databases getting hacked, I'd rather not create any new accounts unless absolutely necessary.
Yes, I get the danger of counterfeit items. I did get bit by that with fake eclipse glasses from China last year but nevertheless, the 3rd-party marketplace has been a net positive for my shopping experience. I just pay attention to the seller ratings and make sure they're better than 98%.