Oh, he's talking about "Arbitrage," the practice of capitalizing on the price difference between two or more markets, and its place in the Team Fortress 2 economy. That makes more sense, barely, but between the specialized equations and graphs Varoufakis offers some plain insight into the world of digital economies.
For example, 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 isolated nature of digital economies lends itself to intriguing happenings such as that one, as Varoufakis explains in great detail, we assume because he's thrilled to be able to calculate the exchange rates of laser guns and hats rather than boring old dollars and cents.