I used to work at a large company that specialized in "e-trading". They paid a fee for access to second order quotes, which meant that they knew about not just the current price of a security, but the actual stream of bid and ask prices from individual investors. If you have access to the stream, you can just write code that slightly underbids and offers slightly overpriced shares, so you get to nickel and dime investors all day with sub-millisecond accuracy. It was basically software that stole money from everyone all day.