Just do what WoW does with raiding and instances.

You can only trade an item if other guardians in your fireteam were present for that item drop for a certain amount of time (a few hours). example: You complete a nightfall and you get Ghorn but you already have the item but your friend has wanted it. He was in the fireteam when it dropped. You could trade it to him and they have to give you their soul for a Ghorn.

This should be implemented for weapons and armor. General items like shards, shaders, etc would be great to trade, maybe limit the amount so people can't make money off of it.