I think the lack of trading, as it stands right now, is appropriate. I like Bungie's notion when they say that they want every piece of gear to have "a memory with it." Plus you might end up with people becoming Glimmer farmers and Exotic loot vendors which would negate the need for most other vendors in the Tower. There could be other ramifications that I'm overlooking at the moment, but Destiny's economy is too new. Time is needed to see how well it works and to squash any bugs. There could very well be a trading system implemented, but the game needs time to play out first.