Honestly... They need to fundamentally change their economy system to stop it. Guild wars 2 has no gold seller because the game is set up in a way that gold seller can't make any money. In that game, you can buy gem to purchase cash shop item. But the gold you earned from playing can be converted to gem. gem can also be converted to gold. And exchange ratio is constantly updated.... So anet stopped gold seller by letting people buy gold from them.