Robinhood, the Silicon Valley startup that operates a commission-free trading app, is opening up a waiting list for customers in Britain. The company says it will go live in Britain, its amke international market, in early Professional trading firms, sometimes called high-frequency traders, buy the retail-investor orders from brokers like Robinhood and execute the trades for. These trading outfits typically make money from the gap between the bid and the offer. When the trading company buys order flow, they give some of that money the rebate to the brokerage that provided the orders. Brokers can keep that money or offer better prices for their clients. And some regulators worry that brokers could be tempted to send trades to the market maker that offers the best rebates, instead of the best stock trade execution for the customer.