I used to live in the People's Democratic Socialist Republic of California (emphasis on 'used to').
Private gun sales are forbidden by law. All firearms must be sold via a FFL holder. For which, of course, they have to prepare documents and charge a fee. Could anyone imagine the uproar if one's used car had to be sold through a licensed car dealer? (Like the one sold on Craig's list and not re-registered that was used in the attempted Time's Square bombing last year or so?)
The 'gun show loophole' is Leftist jargon for 'being able to dispose of one's personal property without governmental permission or interference'. I now live in Nebraska - the Heartland! - and can purchase from or sell to other private parties without let or hindrance. Licensed gun dealers have other requirements like filling out the federal forms and doing checks.
At the same time, if I'm selling a gun, I will not sell to someone who gives me a bad feeling or seems to be up to no good. It is my property, after all. Not to mention my conscience.