A clarification about trading system and coin/gold

As I do not want to be banned or something, I want to understand exactly how trading system works.
I need coins (I spent all those Warmane gave us after server reset, but, sadly, the character I lost would have needed maybe twice those coins to be restored!)
In Trading window I see gold sold for coins, but not the opposite. I have plenty of gold, but I need coins.
So, in short, as it is not legit asking in game or using other means besides Trading system, can someone explain me how can I trade my gold for some coin?
Thanks

You have to put your gold for sale. When you are in trading section click "sell" at the bottom and you will see the options for putting your gold or items for sale on trading.