A trading system, no matter what way they spin it, would result in the death of the game incredibly quick. -No more stocking coins waiting for Xur to come to purchase his weekly stock or exotic engrams -No more grinding nightfall on three characters hoping for that one specific exotic you've been really wanting -No more leveling Iron Banner to pick up the weapons and armor being sold -Why even try going flawless or beating Skolas? That elemental primary you want can just be traded to you -Coveted weapons like Dry Rot for PvE and Hopscotch Pilgrim for PvP dropped from Dragon Strikes? Why bother grinding strikes for an undetermined amount of time, just get one from a buddy! Easy! Seriously, within a month or so every player would have whatever they ever wanted and then some, which would lead to quickly maxing it all, which would plummet demand and rarity of many weapons while also seeing record lows for participation in most activities and events, which leads to people growing bored, which leads to their friends leaving for other games, which results in much of the community quitting or moving on, feeling there isn't much else to do or a real point in doing those things except the players who would still do them for fun. Bungie wouldn't give their game a death sentence that heavy.