The reason Costco is able to do this is because of the "buyer" team of Costco. There have been many articles talked about Costco's secret recipe. I believe Sam's club did the same to some extent though not as good as Costco's team.
Amazon operated on a complete different model, more leaning towards ebay than Costco. Comparing Amazon to Costco is like orange and apple. Amazon's model let the 3rd party seller resolve the market and quality, well, didn't quite work out when met with Chinese sellers who're from a society of bottom line driven all the way business philosophy.
Unfortunately, Walmart has followed Amazon foot steps but fortunately not quite enough to run into Amazon's problem.
Amazon operated on a complete different model, more leaning towards ebay than Costco. Comparing Amazon to Costco is like orange and apple. Amazon's model let the 3rd party seller resolve the market and quality, well, didn't quite work out when met with Chinese sellers who're from a society of bottom line driven all the way business philosophy.
Unfortunately, Walmart has followed Amazon foot steps but fortunately not quite enough to run into Amazon's problem.