Trading allowing you to trade resources with other players and it is useful if you need oil for your tanks, if you are country which produces no oil or a little of it.
Before Resources Update, you could trade only oil for cash. If a player sells a lot of oil then market crashes and oil's price is much smaller. If the global stockpile is small then oil is expensive.
New trading system Edit
The old trading system has been deleted forever. Now, trade system is divided into GUI with your all resources and the global stockpile is deleted. Trade consists of selling resources to a selected country, not as before, globally.
Buying resources such as phosphate and making them into advanced resources like fertlizer can yield a lot of profit,making it potentially the best money making method for small nations.