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
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.