ok, so figure a whole guild is the type to not look everything up online before doing a raid or a quest. how are the supposed to know that this quest item is going to be blue. sure they can research these things before going about it, but is there something in game telling you to beware of this? when it comes to business if you are not properly notifying your consumers of the risks taken in your actions then it is up to the business to repair or replace or exchange the effected item. it should either be mentioned in the quest or have a warning when looting them item stating that it will not be tradable. is the item BoP? if so shouldn't it give you the warning stating that the item will be bound to you with out the ability to trade it after looting it. if it is BoP and the person who looted it just recklessly clicks "ok" without checking what he is agreeing to then it is the fault of the consumer, who at that point was well notified of the results of his actions.
none of this is meant to sound snarky of anything, just relevent questions. think of these things, these questions should help you find the answer you are looking for as to who's fault is it in the end.