Quoting hennnry " What's wrong with this practice? It doesn't make or break MMO. So it's like... you created a service (a gaming service), and why can't others who utilize your service to make a profit?"
You see the companies that builded the MMO have also builded a online market for that game, it helps them keep the servers running, this way these guys not only are they in violation on the rules, but they are also robbing costumers by using unauthorized sellers, wheter it should be legit to have 3rd´s also selling is a different thing. See it his way, if you had a pizza restaurant, would you like if all in a sudden someone came inside your establishment and started selling pizzas, "you created a service (a pizza service), and why can't others who utilize your service to make a profit?" "