Indeed, and it also brings to light the extent to which our economic systems are tied into the idea of the nation-state. Which government would collect taxes on transactions made in a place with no geographic existence?
Our highest forms of political organization become increasingly dated as the nature and location of our lives and activities becomes increasingly immaterial.

“I presume that if these economies are taxed, the form the tax would take would be something akin to a flat state sales tax. ”

I would say it would more likely take the form of an income tax, asking users to declare money made in-game for taxation. I don’t believe it would be possible to implement a sales tax ingame; requiring users or developers to record and report all ingame transactions seems excessive.

Sales tax is a regressive tax; the problems you observe in your hypothetical in-game sales tax already exist with regards to real-world sales taxes.

It seems to me that Blizzard regulates transactions outside of the game world because such transactions run the risk of destabilizing gameplay balances that have been painstakingly developed. The economies of the Diablo games ran rampant and the gameplay experiences became far different than was originally intended.

Second Life is defined by these transactions; the gameplay experience is composed, essentially, of a series of them.

I think you’ll see attempts to further regulate activities of significance on the internet, as they threaten the relevance of existing structures.