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?
Plus the illegal items being sold. Who would monitor those.
Piracy communities have been defederated just because worlds wants to preempt legal trouble, just in case the moderation there slips up.
What more for physical items, such as illegal drugs and our services. The fediverse might quickly find itself to be a clear net version of the silk road, or a real hotbed of counterfeit items.
Lemmy is being moderated like crazy... Maybe even worst than reddit....