How do they get paid? I didn't see that in the FAQ. Presumably they take a cut from the seller? If so, I'd imagine it might be frustrating to negotiate which 3rd party you pick for what fee for each transaction.

Perhaps, but is it that different from sellers selling only on some specific websites/stores and not the ones you want to buy from?

That's my point. I don't see how this would materially improve my experience. I've picked my trusted intermediaries and I shop through them.