You can remove an NFA item from the SBS status. I have removed a couple from NFA staus in the past.That is why I am going to sell the items separately, so there is no violations. Once the receiver is removed from the NFA registery, then I sell it like any other M4 receiver. The barrel by itself does not need to be registered NFA.
If someone does buy the barrel and they aleady have a receiver, they better make sure they do a form 1 on their shotgun or keep the barrel at another location away from the receiver.