Gold Buying on the WoW Support Website has Blizzard's take on gold buying, explaining how it impacts all players (thanks Ant). Here's a bit:

We regularly track the source of the gold these companies sell, and find that an alarmingly high amount comes from hacked accounts. These are the friends, relatives, and guildmates you may know who have gone through the experience of having characters, gold, and items stripped from them after visiting a website or opening a file containing a trojan virus. Our teams work to educate players and assist them in avoiding account compromise, but the fact remains that the players themselves are often these companies' largest target as a source for gold, which the companies then turn around and sell to other players.