When I was still gaming on a regular basis, I came up with the idea that in WoW a simple solution would be the following. Most if not all Gold sellers are using lvl 1 characters, and, most if not all use the words or combination of words, Gold, 1000, $xx.xx and some web address in their shout outs. By combining these variables with the current ability of people in game to tag players as spamming, it would be fairly easy to turn a character into a killable target if it passes a certain spam score. Let's say three spam tags and at least 2 key words. It then becomes a game to kill the Gold sellers.
The other thing would be to make it impossible for any character on a trial account, or under a certain minimum level from entering any city.
When Gold sellers have to pay $15 a month to create characters who have to be played up to level 10 before they can get into a city and keep getting killed the second they shout out, it becomes more of a cost then a benefit.
Oh, and to all you mages out there, and I am one, stop porting lvl 1 characters to cities.