They're required by law to give you execution at the market price or better, see https://www.sec.gov/fast-answers/answersbestexhtm.html https://www.sec.gov/fast

They're required by law to give you execution at the market price or better, see https://www.sec.gov/fast-answers/answersbestexhtm.html https://www.sec.gov/fast-answers/answersbestexhtm.html Because of this, it's legally forbidden for them to mess with your prices in such a way that you lose anything. I think people see that these firms make money from selling order flow and as a result assume that they must be losing money somehow. That isn't the case. Some firms (e.g. HFT) are simply willing to pay a little extra for the counterparty to their trade to be a dumb joe instead of another HFT firm. This can lead to both you and the broker winning.