Mostly a thought experiment, I want to hear the reasoning

The reasoning is that the anti-2A folks have no problem infringing on the right of the people to keep and bear arms. They want your guns, whether it's constitutional or not.

It's nothing more than another example of situational values / ethics by liberals. If the situation is such that a constitutional right is being violated, and they happen to agree with that particular right, they'll scream to high heaven if someone tries to violate it. (as they should, btw)

If the situation is such that a constitutional right is being violated, and they happen to disagree with that particular right, they don't care, and seemingly the entire argument of constitutionality bounces right off them.

These are the types of obvious traps they find themselves in when people with a keen grasp of the obvious point out their hypocrisies

I'm not sure what you mean. IDs are required for a firearms purchase even at Gun Shows. Are you talking about private sales between a gun owner and a prospective gun owner?