Sell an old car for more than you paid — in many countries you pay tax on the difference. Profit from trades works the same way, only every country has its own rules.
What to find out for your country: whether trading profit is taxed, at what rate, whether you can subtract losses from profits, and whether you have to file a tax return yourself if the broker is abroad. Ask the tax office or an accountant. Articles on the internet often go out of date.