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how to process payments? how to ship goods? how to handle refunds? how to handle contestations?
The problems are solvable, but the solutions taken together are couple times as complex as Amazon itself. This translates to cost. Which is naturally the reason Amazon came to existence earlier than that solution.
I think that layers of storage\messages and actual logic should be firmly separated, an instance going down when someone wants a refund for an operation that involved it seems not good enough. If the operation is a cryptographic contract with an escrow, and “instances” are just servers providing message storage probably privileged for some users (might be members of a community, might pay for that storage, that’s lower layer anyway), this is less of a problem. But that’s not a federation.
By the way, however I dislike OP’s attitude, if you suggest this idea like a federated ads and reviews platform, it becomes useful.
They might have run a small business or been present in a bigger one in management position, doing their own job well enough to avoid painful understanding they don’t get it as a whole. Arrogance is not always cured by experience, actually I doubt it’s ever cured in humans and we all have it.
That much was clear from the very beginning, I tend to have such ideas too, but I have BAD and thus mania periods.