This has been suggested many times before and declined each time.
What if people trade items of hacked/stolen accounts? It's a significant amount of work to restore everything.

Not to mention that it's punishable to sell/buy/trade anything FGunZ/forum related on the forum or in-game.
You can receive an infraction (forum), warning, or a mute if you tell people that you are selling/buying/trading something. If you decide to trade something and you get scammed, too bad.

I mean in general I support the idea and I wish there was a trading system, like in Runescape for an example, where both of the sides need to accept the trade so it's hard to scam people but I don't think such thing gonna happen.