As someone who's played Runescape and a few other MMO's in my time, the trading system would fall on its head simply because there isn't enough rarity in the game to establish a proper economy of any kind. There are already so many exotics everywhere to potentially trade, and currency with which it is already very easy to simply buy exotics. In a typical MMO, there are so many activities you just can't ever do, so you need to trade to get good gear, and in game money actually means something so its possible to go and earn enough money to buy these good items and that's an achievement in itself. In Destiny, items aren't worth anything, vanguard marks and strange coins can kit you out in almost the best gear within 2 weeks, and raid gear would not be hard to come by at all with how easy it is to get exotics to trade for it. Trading would only serve to flood the market and ruin the game.