In Texas private sellers can rent a booth at a gun show and sell without performing a NICS check.
And they will need an FFL if they make a business of selling privately.
I think you inadvertently hit on why people are opposed to further gun regulation. The people committing the crimes aren't gonna follow the law and it stands to only further burden regular citizens.
Can you give me an example of how this would work?
Anyways, it's kind of a moot point in California because that's a felony here.