It may seem like it didn't need to be said, but they're saying it anyway: gold buyers are financing the hacking of accounts and the selling off of people's gear. Since we at WoW Insider have seen a lot of these cases (have even had them happen to some of us) we know how profoundly irritating and even emotionally painful it can be to have all of your work on a character or characters gone in a heartbeat, much less seeing them transfered to other servers and even in some cases having their names changed. Blizzard points out power leveling services as a particular culprit in this trade, saying "Through our normal support processes and the assistance of players, we also find that many accounts that have been shared with power-leveling services are then hacked into months later, and all of the items on the account are stripped and sold off. Basically, players have paid money to these companies, sometimes large amounts, and they're then targeted by these same companies down the road."
Now, this doesn't mean that you shouldn't still be careful with add-ons you download or keep a good anti-virus program on your computer, as we know there are many malicious trojans out there targeting World of Warcraft players. But just as clearly, if you use a power leveling service or buy gold, not only are you funding account hacking, you're in danger of seeing your own account hacked as well.