There are also retailers which actually vet brands. For example anything edible that Target or Walmart or Trader Joe’s or Whole Foods (Amazon ownership notwithstanding) sells in their stores, they actually contract a testing company to test it to make sure that it’s safe. For things where safety is important an actual physical national store is going to be much better than Amazon.
Yes — but one retailer can aggregate hundreds of manufacturers. So if you buy everything at Target, for example, you know they have done a basic-level quality filter for you: the brands are not counterfeit, the food is tested occasionally, the electronics are UL listed.