The funny thing is, it's not really bilking the stock market. The whole argument for stock trading is that traders create value by accurately pricing securities, and thus allocating capital efficiently. Time travel is just a ridiculously efficient means of doing so. Given common access to Time-Turners, the stock market would literally be perfectly efficient(assuming that using turner-induced stock prices doesn't violate the 6-hour rule). People without them would be very pissed off, but I'd actually argue it as being the right and proper way to run a stock market if the technology existed.