The agency that oversees Wall Street is weighing major changes to the way millions of everyday investors buy and sell stocks. That could be bad news for so-called free-trading apps like Robinhood as well as the lesser known firms that underpin their business models.

Today, when you buy or sell a stock on an app, the trade appears to be instantaneous. But beneath that simple buy/sell action is a complex web of Wall Street players exploiting tiny differences in price to rake in huge amounts of cash. READ MORE