Can he explain exactly how this is supposed to happen? Let's think it through. You see a stock priced at B, and you decide you want to buy it. Cuban is saying that a high frequency trader will see that you want to buy the stock, and he'll buy it for B and then sell it back to you at B+X, making X in the process (any you pay more for the stock). To do that, he'd have to know that you were about to submit an order. But he only finds out about your order after you've submitted the order to the exchange, and the exchange has relayed it to him. He doesn't get the opportunity to "jump in front of you", and I have no idea why Cuban thinks he does.

In actuality, what happens is this. The high frequency trader sees that the stock is priced at B if you want to buy it, and S if you want to tell it (with B > S). He then expresses his willingness to sell you the stock at B-X (by sending a limit order to the exchange). When you come along to buy it, you get it at B-X instead of B, which saves you money. The trader gets a rebate from the exchange for supplying liqudity (which he did, since you got the stock for less than you would have otherwise) and hopefully he is able to buy it back for some price around S (with another limit order), thereby making the difference between B-X and S.

There are dangers with high frequency trading [1] but they are not of the kind that Cuban is describing.

[1] http://en.wikipedia.org/wiki/Knight_capital#2012_stock_tradi...