Gold selling is a problem of game companies, because it lowers their revenue. Selling gold is against the TOS. Legality is not an issue.
Many drugs are illegal, which makes them a problem of the state.
In a game like Wow one person selling gold to another isn't lowering thier revenue since Blizzard isn't selling WoW gold.
Considering 1k is about 1.0$-1.2$ depending on a realm ( just looked ) and average person can make 1-2k/hour its not much. So I think behind gold selling ( China and so on ) is Blizzard, for them it's not hard to generate random amount of gold to a specific desire. That is why I think gold selling is against the rules, since you are taking their customers...
But again, i might be wrong.
You are correct, they do not like the sales of "THEIR" virtual property, but knowing Blizzard and how profit hungry they are I would not be surprised if they had some sort of correlation between the online sales of Gold. Plus if you get banned for buying selling gold, most likely you will be a gamer that will be willing to purchase another account and not make the mistake again, also increasing their sales, if they wanted to shut down the Asia markets for MMO-RPG games currency they could by easily undercutting the markets and causing the companies to foil. Speculation~