Well...in California, all sales are required to go through an FFL, no matter whose selling or buying. Other requirements and exemptions apply. Then unless there is a law enforcement type of exemption, there is a 10 day waiting period and background check.
Private transfers are within the State only. Outside CA. sales of handguns must be on a State roster. And of course an FFL must do the transfer.
That's why so many out of state sellers don't want to mess with CA. The gun in an auction or dealer, must be on the Roster. If not...it goes back.
The roster is a limited list of State approved handguns. Manufacturers submit their handguns for approval. And it it very specific to that the exact SKU that appears on the list, barrel length and finish all apply. There's about less than 100 S&W's on the list The approved has a time limit. Guns go off and on this list throughout time.
So, I can only buy new rostered handgun or used handguns sold through private transfer from within the State. If I want to buy a used un-rostered gun out of State...I'm out of luck, unless I break the law and the seller doesn't care.
There are those individuals who would do a deal outside of the CA. law, regardless of it being a gun show or not. That can take place anywhere.