Gold can't be 'soulbound', but a lot of valuable items are. Also, the player can't really control wether it should be soulbound, it happens automatically. For example, some items 'bind' on pick up when you loot it from the corpse of something you just killed (BoP), some do so after equipping them (BoE). Therefore the chances of finding an unbound item on a player that would sell for a lot of gold on the Auction House isn't too big (unless he crafts them with the intent of selling).

All items (including bound ones) can be sold to NPC vendors however which will yield a relatively small amount of gold. I imagine that these people hacking WoW accounts will just 'vendor' everything anyway to get as much gold as possible.