Selling Arcatraz Karazhan 3rd key fragment

Hello i have a question regarding selling the 3rd key fragment for the karazhan quest. Apperantly this is not allowed however i want to ask you why. If you clear all trash with a normal group and then use a warlock to summon in people to complete their key fragment why should this not be allowed? Lots of guilds have done this to attune their players, i don't see why it would be any different for selling it. Could the GM team give some clarification on this matter?

Thanks in advance!