Wouldnt work. The rng system would have to be completely reworked to keep players on the grind. Items would need to drop from specific enemies. (Like raids) with a way higher drop rate. Now your grinding the same strike/mission over and over. Xur would no longer sell exotics. The rng system now makes it to where your not farming the same activity. Its also set up so you obtain exotics easily. Getting all is where the grind is. With a trading system drop rates would need to be so low that the only way to all exotic would be through trading. This would make harder obtained weapons something a select few would have. Main point. It to late to change it. Its would be a flooded market, exotics would have to trade value.