The key difference is that Amazon is selling its infinitely deep platform to third parties. It is those third parties that bear all the risk of developing and marketing a product. Costco and Wal-Mart buyers and merchandisers still need to take the risk of choosing what limited inventory to stock on the shelf and commit resources to making it happen. Amazon also invites all competitors in a product segment to compete with each other. Using the information gathered from the sales, reviews, page views, demographics, and who knows what else, they can create the perfect Amazon basics version of that product pretty much on the fly with pretty low commitment.
Sure it is. Stores rent out space on shelves to manufacturers all the time. The manufacturer even stocks the shelf.
And the store gets to pay them only 2 to 3 months later on top of all of that.
Even when stores buy stuff and stock it themselves, they don't take any risk on it. Unsold stuff gets returned, and they only pay once the merchandise is sold.
Because Costco are taking a risk. You can only have X different widgets on a shelf, its a limited resource so you can't just let every brand in to sell their wares and take a cut. This in turn means Costco has to take on other risks. They're liable for fakes, they're liable if something doesn't sell (I assume Costco probably does sale or return for new suppliers? But there are limits to this). They're doing the fulfilled by Amazon role as well.
Its the difference between a Farmers Market and wholefoods. You as a consumer probably have nothing to do with the market organisers, and they don't have any particular responsibility toward you.
