I think that with the state of Destiny's "economy" right now it just isn't a viable option. Over half of the of the loot in destiny is trashloot that doesn't really have much of a use anymore since upgrading weapons got revamped, leaving a select few of strike and crucible drops as real "loot". in essence, trading would amount to a couple of "real" loot drops being thrown around whilst guardians cycle exotics and blackmarket vendor weapons. It's kinda sad that the item progression curve is so jacked; in the beta I'd be so excited to decode a new engram and check out my new guardian's fashion, and I could see the trading system make use since people would like to get the different models of the same armor piece. A real shame that the "end game" gear is pretty much where the whole experience is and thus trading would honestly be pretty repetitive for such a compact market. Edit: Seconds after I made this post I remembered that items randomly roll stats. So, weapons would have a lot of use since there's such a high demand for certain guns that roll certain perks. Armor if people want to min/max stats. Still though, my previous points still stand.