There’s the theory that with perfect competition and perfect information, in a standard one item model, seller would set price = marginal cost and social surplus is going to be maximized. A similar intuition explained using general equilibrium has been given in an Arrow paper.

But nothing is perfect, right?

I think the story is quite simple, though not necessarily true: a large selling platform can use its market power to force sellers equate their price across channels. But because the platform charges a large comission rate, sellers would have to raise price uniformly on other cheaper channels too.

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