This is my thoughts on how to implement trading into destiny.
1. Raid items cannot be traded .
2. Items scale down to the power of whatever you have equipped in that slot .
3. Planetary resources can be traded .
4. Silver and bright dust could possibly be traded .
5. Engrams can be traded .
6. Only weapons and armor from non end game activities can be traded .
7. Glimmer can be traded .
8. Quest specific items cannot be traded .
9. Emotes can be traded
10. An looking to trade emote could also be implemented