Good deterrence signaling would make plainly, soberly clear that the US would treat a weapon with DPRK-originated fissile material as originating from the DPRK itself. The DPRK leadership does not want to die.
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Separately, it should be noted that no state has actually ever sold nuclear weapons. (Enrichment technology, yes. Missile technology, yes.) Part of the reason is potentially attribution issues, but another part is that few have enough weapons to feel comfortable selling.
I think the idea that the DPRK would find it appealing, or easy, to sell full nuclear weapons is... somewhat imaginative. But it is also an idea that can be dealt with in non-military options (like improving detection of clandestine movement of materials, for example).