So it's like a dark pool (in the capital markets sense) for retail, in which you try to undercut the market maker's (Amazon) spread by narrowing it yourself and

So it's like a dark pool (in the capital markets sense) for retail, in which you try to undercut the market maker's (Amazon) spread by narrowing it yourself and not telling anyone about how narrow it is? It's an interesting idea, but I don't think it would work because (as many commenters here have already said) Amazon's spread is already quite narrow.