As I see it, I would just like to recommend that everyone trades their weapons IN-GAME aside from giving a stranger their code as this is A LOT more safe instead of giving up your code.
Sadly, these won't be 'traded' (they won't even show up on the trade list) with other players, and you won't be able to 'dupe' them in game either. These are 'DLC', which means that they will have additional restrictions tied to them (i.e. you won't be able to use/have them on a secondary console if you aren't logged into Live). Also, according to a Lionhead post, these exclusive items will NEVER be available to purchase on the Live marketplace.Exploits my dear friend, exploits.