Sometimes there are profitable trades, but not that often. Sometimes someone is looking to sell 100 widgets at $20 on the American Widget Exchange, and someone else is looking to buy 100 widgets for $21 on the National Widget Exchange. And so you can buy on the AWE and sell on the NWE and make an instant $100 profit. But only one person can do that. Once you buy on the AWE and sell on the NWE, the window is closed.Who gets to do the trade? The most intuitive answer, in most cases, is “whoever do...