Someone's said it before you should only be able to trade if you're playing together and an item drops for ex: friend gets fatebringer I get praedyths revenge he offers it to me I accept it and trade the weapon that dropped for me for something that dropped for him. Knowing how some people will do anything to make a couple bucks they'll still find a way to somehow exploit this but if it's made into a sort of snap decision type of trading it will definitely be harder and won't stick as a way to monetize in game trading