It might sound weird, but right now somebody are trading not money, but various in-game items that became equivalent to money in some areas. These areas are CSGO skin trading websites such as Drakelounge and others.
The need of virtual currency
Each online (and sometimes even offline) game has their own currency, whether it’s gold or bottle caps (like Fallout), or skins in the case of Counter Strike. These currencies allow players to not only feel like more involved in the game, but also to be able to participate in gambling and eSports.
The rise of eSports is one of the key features that allowed skin gambling to grow to the level that is now. After all, the ability to gain some more money attracts more people. Those people then start playing the game professionally and so the bets grow and more people know about the game. So in a sense, it is an unlimited supply and defense loop.
Of course, skins can be traded not only in Drakelounge but in Steam Community Market, which also brings them closer to real life economy.
You could say that the virtual world’s economy is no different than we are in. After all, it is controlled by supply and demand, there’s gambling and various opportunities to earn more than you got.