Having a trading system could ruin this game
How could it ruin it any more than an NPC that sells random exotic loot for easily obtainable coins?
Today i've done the raid 3 times on hard, done 3 nightfalls and done the three weekly heroic missions. Thats an awful lot of loot chances. I got an icebreaker, a mythocloast, 2 raid helmets, raid gauntlets, raid boots, found verdict shotgun, praedyths revenge, the calming fusion rifle, the exotic warlock chest piece and a dr nope.
How many of those things did i have already? All of them. Was that a complete waste of time and effort? Well yes, it was. Now i have bits that my friends really wanted and i have nothing to do with them except dismantle them. It annoys me and it annoys the players who wanted these things, that saw me get the item yet again. I'd like to be able to trade with the friend that has 2 gallahorns or just give away one of my fatebringers to a friend that has been wanting it for a while and never has it drop. Hell even a big machine in the tower that takes in 5 weapons and smashes them all up and spits out another item of similar level in the hope i get something i'd like.
Any time a weapon is said to be too powerful it gets nerfed so that all guns are basically as balanced as the next or a hell of a lot lower and with the mass distibution system (aka xur) it wouldn't matter anyway how many other players had these things.
The only way this would ruin the game is that people would get the things they want, then wouldn't have a need to play the game anymore. Bungie want them to keep playing and keep buying the dlc's and doesn't want people to trade the game in at the shop so they don't want people to be able to trade.