I'm going with "bad idea". Initially, I thought a trading system would be good to have, but I've changed my mind. There's not enough weapon variation or challenging end game content. So if you can let players trade to obtain all of their desired gear (I can totally see people selling these items using paypal), there would be far fewer high level guardians grinding day in and day out. Which leads less people around to join randoms for nightfall/weekly/raids. Xur is bad enough with regards to making exotics purchasable (don't get me wrong, I buy from xur too because it's a way around the RNG loot system). Just my 2 cents, trading will hurt destiny.