Why do players feel they need a trading system? Go straight for the underlying problem instead, which I think is how loot is rolled and distributed. Fix THAT and there won't even be a need for trading. I'm not going to say Diablo 3 is a perfect example, but you can't trade legendarys in that game any longer, and tbh I've never felt the need to trade ever since they fixed their loot system. I'm mostly playing solo so not even trading with others in my group when an item drops.