Buying from an alternative ecommerce site usually sucks: you have to register for every website, enter your address, payment information and other information, they may leak data or store it improperly, you may not know the reputation of the website or business, you can't easily compare products with other vendors and more. Amazon and ebay offer a centralized good experience and you know you can trust them with your purchase. They benefit the consumer by aggregating many businesses so it fosters competition lowering prices but they have so much power and they have done some anti consumer moves. Their fees could also be a problem.
The same way mastodon offers a viable alternative to the deadbird platform and slice power to small instances while getting a better user experience. (And lemmy to Reddit.) A fediverse version of ecommerce could perhaps be viable: federated ecommerce that aggregates small business shops, handle the user details and let the business access it when you hit buy. Activity pub to communicate the listings and purchase orders. I am not a programmer and don't know the technical implementations of it. So what do you think?
Oh, so Shopify + Stripe but decentralized. It sounds... It might have some security issues, and of course banks might offer some resistance.