The most interesting extension of this is the example Lyn gives of monetization of that magical item in that online game
It was a rare (but not too rare) and universally useful ring in Diablo II. It had more favorable properties than the in game currency which the designers had tainted with conditional durability.
virtual economies
This book goes beyond in game currencies and explores how these currencies sometimes "escape the lab" and begin competing with sovereign currencies as was the case with QQ coin.