Costco == Trust.
Trust in quality, Trust is service, Trust in prices.
To the point that when I need to buy quality stuff at good prices and avoid doing lenghty research to filter out scam, fakes and crap (aka: Amazon, Ebay and their little siblings) - Cosco wins.
There is no other business where people come in with an intention to buy $50 worth of stuff and then consistently spending 5-10 times more each and every time.
Fun fact: It took me 10 years to stop calling it "Price club"