[frandroid]

Brokerages pass on trades to market makers. They don't promise a price, they just tell you which price is available through market makers, and when you decide to go ahead with a trade, they go on the market, and you hope that the price market makers were advertising earlier is still what they're offering when you come through. Brokerages make a profit by passing on your order and taking a percentage of your total sale, not by making a profit on the spread.