The only problem I see with a trading system would be people abusing it.
I fee that if a trading system is implemented, it should be either unlocked at a certain level (20+) or only items that are not usable to your class.
Another method could be clan bases within the game. But Ill explain and post that method another day when im not feeling lazy. The way it's put together sounds much better than simple Clan Base. lol
Yes I understand that, it's easy to exploit this kind of systems (almost every mmorpg out there has a broken trading system because of all the players exploiting). But as you said, a level requirement and item restrictions would help, it would be nice to share shaders, sparrows and jumpships for example and leaving legendaries and exotics out of the system