I think this might be Shopify's play, at least it would be what I would do if I was them.

Enable high quality sellers to have their own branded website to sell their own product directly.

Then make it easy to find the Shopify sites via some general search or directory.

You're able to slowly build up power this way without it being too obvious, and you provide an immediately valuable service to sellers that aligns incentives from the beginning.

Shopify has Shop app now. And some sites have integrated Shopify pay as an option to pay. I assume it’s Stripe white labeled.

Unless they're hiding it well (always a possibility...) Target is that retailer for me. Of course they don't have anywhere near the selection of Amazon, but for the regular essentials I need their free two day delivery does a fantastic job.

IKEA.

I could name a hundred niche sellers too, but the allure of Walmart and Amazon is that they sell basically everything. I can buy a Macbook and Gatorade plus some brand name AA batteries for my RC car that I bought all in the same shopping trip.

IKEA has good products but they don't come close to general merchandise stores like Amazon or Walmart.

What about Costco?

Costco is the opposite of Walmart; they're more like Trader Joe's. They sell hardly anything. The idea of Costco is "if you find it here, it's a decent product", not "if you want it, you can find it here".

Target?