The problem is instead with the shell game these vendors are playing — creating thousands of temporary brands, pumping them or "brushing" them with fake reviews, and then discarding those brands at a moment's notice when things go sour for them, only to replace them with another brand selling the same shitty products the next day, with the same people behind it.

Amazon's seller reputation system was designed to function under an assumption of "one persistent brand per group of people who work to sell a thing." But people noticed that Amazon uses "one legal company with a trademark" as a proxy for "one group of people"; and so have created thousands of distinct "legal companies with trademarks" with the same group of people behind them. Which Amazon's reputation system has no way of coping with.

If Amazon could deduplicate brands — i.e. require that the same group-of-people sticks to selling stuff under the same company/brand — then there'd be no problem, because then their reputation system would work: if the stuff was crap, the company/brand would get a bad reputation, and then nobody would buy their stuff any more.

I don't know how common that is, but buyers should be aware that many Amazon sellers, including domestic ones, operate in the way the OA explains.

Biggest annoyance IMO is that they clog up the search results and make it harder to find products you'd actually buy. Many sell the exact same item, which you then have to skip over, over and over again. Someone at Amazon must be bonus'd on adding new vendors.