This is called payment for order flow, and trader-blogger Jeffrey Carter explains the problem.

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Not surprisingly, the established exchanges would like to ban payment for order flow, or at least make sure the customers know what is going on.

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The main difference being that the more traditional model, featuring pro-rata allocation and collection of payment for order flow, is more geared towards one facet of the investing community, comprised in large part by retail investors who are using options limit orders and other very simple strategies.

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