I believe certain items should be tradable, and some items only being traceable at specific times. Anything up to Legendary status should be tradable but only if it is unleveled and has not been re-rolled. With the exception of raid weapons and armor, that should only be tradable at the end of the raid, with members of the fireteam. Exotics are already pretty easy to obtain, but I do see the same exotic drops week after week... So instead of allowing trade of exotics, implement a trade-in system with Xur. You could trade in an exotic you didn't want and some strange coins with glimmer and receive a random exotic engram or an engram of the same type. (Trade in extra Gjally receive exotic heavy weapon engram..)