The biggest problem, which we should learn well for the real world, is all currency in online worlds are based on pure fiat. Simply put, there isn't a limit to how much currency exists in the world. It eternally flows in, this by itself breaks the entire economy, as it broke the real world one.

That's the foundation problem, an unlimited source of money. Find a way to limit resources in a game world and the economy then can cull many of the problems like rampant inflation and gold sellers. Gold sellers can no longer just sit by and grind it away.