The downside of studying economics in Diablo II is that it's an entirely non-coercive environment, whereas real-world economic behaviour is essentially defined by coercion. That said, many economists are fully capable of ignoring the compulsive aspect of economics regardless, so perhaps for them, Diablo II is a perfect microcosm.
Anyways, even if it is non-coercive in the sense of having no West Coast Code or legislative coercive forces at play, you can still learn a lot about monetary economics in such environments. This author is interested in the emergence of monetary exchange, and that's a pretty big literature for instance, going back to Menger's "On the Origin of Money" (though Menger does note the later imposition of standardization from political authorities, etc.) and more analytical work such as Duffy & Ochs (1999).
This literature on the emergence of currency/monetary exchange from barter exchange typically understands such emergence to occur independent of any such force - all you need is the existence of the problem of double coincidence of wants. What I have in my inventory may not match what you want to trade for in your inventory, so some object that is marginally more "saleable" in the sense of being more widely desired, easier to carry, etc., will emerge as a currency standard. Also see Radford (1945)'s study "Economic Organization of a POW Camp" where cigarettes emerged as a common currency.