It's not that important but just so it's out there. I wouldn't worry about it too much, but here it is;
Copper coins, worth very little.
Silver coins, worth more.
Gold coins, worth a lot.
These take the appearance of a basic smooth coin, no engravings or anything.
The trading system is prominent so even if you don't have any coins, you could chance it with offering an item of relatively the same value. Rare ores and crystals in general are popular.
I honestly don't care, as long as no one starts using dollars or something.