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I think the closest you can come is open source an entire business, from business plans, architecture, systems, payment processing, etc.
You are completely misunderstanding what Amazon actually does, and why it's successful, despite being a shitty company. It's first and foremost a logistics company. People can order "stuff" in many places, but if they order it on Amazon, they'll get it by tomorrow if they order it before midnight. They got warehouse everywhere. They do (some) of their own final deliveries for anyone close to those, use the big logistics players for the rest (ups, DHL, ...) while having massive volume and the power to dictate price that comes with that. The number of workers in the warehouses is actually minuscule for their size, it's all automated. Huge up front cost, very low cost once it's actually running.
Consumers go there because they can get literally anything. Again: warehouses. It's also a market place but that only works (these days) because it's THE place the people go. The reviews are also a massive point, and would be inherently untrustworthy in a federated version.
How would you ever get anyone to go to your federated version for shopping that sells like "some" things? Even if you manage to combine all those shops, you'd need a way to agree on what an item is called (or how to assign id numbers) so the same item from multiple sellers is grouped in the same offer, and many similar small things you take for granted it didn't even ever see/notice on Amazon.