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?
This is kind of like what happens internally on platforms for 3rd party sellers like eBay, Amazon, and AliExpress. Even decades later they're still working the kinks out obviously. Amazon and AliExpress particularly have lots of scammers, so they clearly haven't figured out the secret sauce yet. They're not under-resourced, so either they're under-motivated to weed it out or it's actually pretty tricky to do.
My guess is it's both, but more that it's just tricky to implement a reliable system of reputation and trust. EBay and Amazon got around it early on by being cheap and establishing policies that heavily favored buyers in disputes, which made the prospect of using the service less risky to the public, improving their market shares. They probably also have non-trasparent systems for tracking buyer reputations as well to avoid abuse.
It seems to be the norm to keep these systems obscure to avoid abuse, but to make a truly functional open platform you would need to have public systems, so I'd hope that the norm of obfuscation is out of convenience or laziness and isn't required to make the system function.