EVE Online's Fight Against Currency Farmers
Massively has a writeup discussing the way CCP Games is battling ISK-farmers in EVE Online (ISK is the game's currency). The developers felt that merely banning sellers whenever they could was not enough, so they introduced a system where players could purchase game-time codes that could then be sold within the game to other players. Since players are unlikely to give up buying ISK voluntarily, CCP's thought is that they can at least keep the money and currency distributed among the real players. Some of the player-base has been critical of the plan, but it's becoming more and more popular as time goes on — and the old ISK-sellers aren't pleased.
I often see the annoying spam from ISK sellers, and their stuff is more expensive than the current cost and return of game time cards. I hope they die from this game soon. I know a lot of people who spend extra time making money so they can play for free (supplying the ISK in exchange for the GTCs). It creates a nice exchange system.
A 2 month GTC will cost you around 600-650M isk. With a proper setup and the right skills you can easily make this within 12-15 hours (2-3 days of semi casual playing.) - The way I look at it is that basically you're working for 12-15 hours and the pay you get is $30, which isn't exactly impressive if you compare it with other jobs (i.e. if you take a weekend job every other week and use that money to buy play time.)
Still, if you don't have the money and you do want to play the game, it's a nice way to keep your account(s) running. I definitely think that the GTC trade has made things less interesting for gold miners and that's a good thing. The Eve economy is good, in fact better than most other MMORPGs I've been playing.
I don't know that much about WoW, but this sort of market manipulation happens in EvE as well. But it's usually not as big a deal, because the economy is so decentralized. If someone's relisted all of the red widgets in a system, there's a few thousand other systems I can look in. Plus because players have so much control over the production of most items in game, producers will notice the relisting, and will increase their production of that particular item. It self-corrects pretty well.
One time I threw a brick at a duck.
What you describe is the old system.
In the new system, every 30 days of GTC time is converted into a PLEX, an in-game item. The in-game item can be sold on the regular in-game market. When it's used, it adds 30 days to your account.