HAVE U EVER SOLD ANY COMMODITY WHICH YOU DON'T HAVE ???????? guess
If you feel that current oil price is too high and its price can lower in coming days as per market condition, you can sell oil without buying it first and when market oil price comes down you close the deal. The difference of price at the time when you sold and buy it back gives you profit margin which is your earning. .
Starting business by trading sell order or buy order ,it is your choice ,and your trading along with other traders buying/selling pressure sets direction for that market price. If your trading is positive with market moving that means you are in profit.
These type of tradings give price discovery for any product. And trading is known asbased on margin trading.