My initial idea was gold and silver rings, as precious metal is the oldest currency system around. But that poses a problem as well. People need to be able to buy items as small as a bullet or an arrow, or as big as a car. A silver ring is pricey for a single bullet, and how many bullets would equal a car? 20,000 or more? Where would all those rings come from? Bullets would be a utilitarian choice, but then that adds the complication of people needing to load the game's currency into their gun and fire it.
Anyone have any ideas? I guess at this point I am leaning towards "gold bits", without really elaborating on what those "bits" are, but they would presumably be small. But I am still looking for something better!
