There are some interesting thoughts at PlayNoEvil regarding gold farmers and buyers, and how best to deal with them. One potential solution? Visibly, spectacularly, and seemingly randomly punish people buying gold - to hopefully discourage people from buying, disrupt the flow of gold from farmers to consumers, and make repercussions for buying gold random, retroactive, public, and aimed at damaging the whole system (not just an account - or thousands):
The more I have considered this issue, I think spectacular, arbitrary punishments are the best penalty for gold buyers.
I'm leaning towards "The Roll Back". The game operator detects a gold buyer. He reviews the account file and makes a notation of where the player was when he bought the gold (or farther back, or at the time he is detected). This state is quietly saved. Then, at some random date in the future - say, 1 to 6 months later, the player is notified that he was busted for gold buying and his account is rolled back. No gains, no experience, no nothing from the time since counts.
This should be done rather publicly on a daily basis... banner headlines - a Player was rolled back from Level 63 to Level 20. He lost X gold, Y experience, the following items... One of those annoying news tickers (with RSS feed, of course).
He's got some more interesting thoughts that are worth a quick read through. While such a system seems fraught with potential technical challenges and ups the amount of effort that must be invested by the operator, it's not like companies aren't spending any time going after farmers/buyers - and what better way to stem the tide of RMTs for virtual gold than putting a punishment system in place that punishes not just people you actually catch, but the whole system?