So you mean you'd come with dollars, for example, buy bitcoin, and then sell it right away, as it'd be falling, and what next? Would you buy a cheaper bitcoin at a minimum to sell again in the next fall? Where will profit come from if bitcoin is constantly falling, and the dollar rate stays the same? Now, if you had originally borrowed some amount in bitcoins, sold them in the fall, and bought at a lower price, then you could get the difference in bitcoins and then in dollars after the conversion. This is the deal mechanism that broker charges the commission for.