In a game like this, I think trading would only narrow the scope of Destiny. People could choose to sit back I'm Old Russia, farm money, and then head to the tower to buy their weapons from other guardians. In essence, players could become geared for pvp and raiding without actually experiencing a large chunk of the game. It also drastically deteriorates the amount of awe that certain weapons could inspire. One person could spend days (game time) earning a weapon like Red Death. But another could buy it. This would be a world not worth playing in. I support Bungie's decision on this one.