There isn't enough content for trading, that is the only reason we don't have it. If we did, it would cut playtimes in half, because everyone would get what they actually want, rather than what RNG dictates. People complain about the RNG and lack of trading, but they don't understand that Destiny needs those elements to keep people grinding that one exotic they want. Think about it, what other "MMO" do you know that had less than 30 top tier weapons? or only two sets of gear that let you reach max level?