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.

Everything you’ve claimed about Amazon applies to traditional retailers.

He's referring to Amazon marketplace. Providing a platform for third parties isn't done by traditional retailers.

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.

Why not? When Costco offers its shelf space to non-Costco brands, how is it any different from what Amazon does?

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.

And Costco takes primary responsibility for all its products, even if relabeled, and has a generous return policy.

Do you really think Costco eats the cost? They force the manufacturer to take it back as a condition of doing business.

AMZN is GLOBAL, Walmart++ need to buy real estate, get licenses etc etc to open new stores. Amazon is a click away.

But yeah, Walmart does the same as Costco: want to keep selling to us? Lower the prices. If they don't sell to Walmart they go bankrupt in x years, if they sell at a loss, they go bankrupt a little later. https://www.fastcompany.com/47593/wal-mart-you-dont-know

Walmart is also one of the largest ecommerce players.

I was thinking the exact same thing the other day when I was listening to NPRs Antitrust series. The talk about this for a bit but the main difference is that Amazon pitches itself as a platform/portal. Essentially any company can it as medium to sell things. It's anti consumer since they are now selling their own brand while promoting it on this so called portal. Costco on the other hand is a whole seller. Companies work with contracts and agreements with Costco to arrange for the selling goods. On top if that, Costco does not explcity promote their goods. If you look at salts, they are equally placed for you to select and the customer can decide themselves.