It would break the game. Think about for a moment and reflect on what people complain about most, next to the lack of story. Content. The whole point of this game is grinding out for hours to pick up exotics. If you brought trading into the picture and then why bother with the raids? The best armor would be available for you to trade... all you need is a friendly high level. And what about that coveted exotic? Yes, the RNG on this game can be very frustrating and at times however it is one of the reason we all go back to the grind every day, every week. It plays upon our natural instincts... rewarding us for playing one more game. Take it away and then what ia the point anymore? Yes, Xur comes every week and practically givea away exptics for next to nothing, however there is no guarantee that it will be something you want or need. TL;DR: Trading would essentially ruin the game and render it pointless.