Valve has hired economist Yanis Varoufakis, and he has a blog, where he recently looked at currency in Team Fortress 2, or rather, how people aren’t just using keys to get stuff. There’s a sophisticated bartering system, and you can actually end up with more than you started with if you trade right.
Varoufakis calls the TF2 barter economy “peculiarly sophisticated,” noting that Steam’s support system allows players to side-step the introduction of currency, operating solely on a “double coincidence of wants.” This is something that civilizations for centuries have not been able to support for any protracted period of time, and its complexity is the reason we now have money, rather than things to trade with.
The Team Fortress 2 economy is far from equilibrium, enabling players to buy low and sell high. For players tired of running, building, and shooting, there’s always the economy to play.