Anyone who sells in-game gold for real currency (€, $, etc.). Sometimes extended to cover someone who sells accounts or virtual assets (items).
How to kill a goldseller as a shaman
There is a quite well known way to kill a goldseller. You have to be a shaman at atleast level twenty-two. Place a 
[Fire Nova Totem] close to the goldseller. Logout when just before the [Fire Nova Totem] explodes(when there is 1 second left, or so.) The totem will turn neutral and blast everthing nearby. A level 1 goldseller does'nt have a chance.
How to kill a goldseller as a hunter
Invite the gold seller to group, this is a must - some will accept the invite thinking that this means you are willing to buy gold, etc. Places in Ironforge and Ogrimmar have neutral cenarion circle mobs. Turn on "Able to Attack" toward the cenarion circle, misdirect the gold farmer and shoot the cenarion druid. Feign death also helps.