You simply don't make a trade for your whole capital sir This is true for every business. If you are profiting on spread and buying/selling currencies then you are fighting variance by having shitload of money. On the average the scenario you have mentioned will have such an outcome about half the time, the other half buy it back even cheaper and substantially increase your portfolio. You need a stash of cash, and time. You keep both BTC and fiat and simply trade it. You can use simple rebalancing techniques in between to keep your desired fund allocation.