Most of the things my NPCS have are bought with gold. I don't know how rare gold is or if only certain types of mobs drop gold coins. How rare are gold coins? Are they only dropped by certain mobs?
Gold coins are pretty rare in the early game but can be fairly easy to come by later on in the game. Some mobs will drop them including bosses but mainly you're just going to come across them as your accrue more and more copper and silver when you consider that:
100 copper = 1 Silver & 100 Silver = 1 Gold
on a rare note slimes will sometimes drop random items including gold coins but I think most of those random items would be ores including iron and gold. But the other guy was right it's like our currency today copper being a penny even looking like it silver being a dollar (silver dollar/dollar coin) and gold being a 100 dollar bill in a coin form and platinum being 1000 dollars.