As much as I like the whole idea of trading it just wouldn't seem right, most players who play crap loads would just end up with every exotic weapon in the game, I myself have 3 Suros Regimes, 2 Plan C's, 2 Thorns, 2 Patience and Time and 2 Invectives and on top of this I have the Gjally and Ice breaker, so I'd imagine if there was a trade system if potentially end up with most of the exotics but that would make things too simple and take away something from the game