Currently in diablo 3 you can only trade legendary or set items to players who were in the game with you when it dropped. I don't see why this couldn't be implemented for destiny as well. I have not seen one CE helmet on any of my characters. Not one. Not ever. My friend gets one nearly every time we complete it. If he could trade me one that he received while we were both doing the same activity together I don't see that as particularly game breaking.