If trading was implimented it would have normal MMO restrictions. Rare, legendaries and exotics would be bind on account or bind on character, so it would come down to trading useless items noone wants and in essence, would be a complete waste of the coders time when they could be focusing on way cooler things. I wouldnt mind seeing an AH in later expansions when I am sure we will have a greater need for commodities like plasteel and spinmetal.