Not to mention the fact that the items they're selling were found within the server drop files themselves, keeping the existing items into circulation.
The problem with selling actual items through gameshops is it throws the economy into chaos as nobody knows what's worth getting any more, plus it only serves to devalue the players time and effort who hunt bosses for their items.
Keeping gold(or X currency) useful so that it remains a valued trading tool is vital to an MMO's success, which is why Bots are being dealt with constantly.