This isn't true and not sure how it even came to be a misconception generally accepted idea. There are couple of interview with the CEO that debunks it. Costco slaps on 15% margin on all products and nothing more. That's the model and is why most big box can't compete with them. Should be couple of interview videos Jim Cramer did that explains this better than I ever can.Costco makes most of its profits from membership fees.
I definitely agree some products on Amazon can be shady. This said still better control over this than what BABA has on its pages.