So, the brokers who have Level 3 can see all the b

So, the brokers who have Level 3 can see all the bids and asks on all exchanges, and when the computers spot a little arbitrage, they can sell it short to someone at the higher price, and then buy it at the exchange where someone is selling it for less. They're trading usually for small amounts, but since it's all done by algorithm, it doesn't cost them anything except the cost of electricity to do this.