Since out of production guns are not on the States Approved List - you can only buy them from a private person either face to face or on consignment.
This means if a gun store gets a used colt mustang they can only sell it out of state since they are not a private person.
It also means you cannot sell it within the state and mail it ! Say you live in Sacramento and want to sell it to your friend in San Diego but you don't want to make the drive. If you mail it to an FFL holder to make the transfer it becomes the property of the FFL holder (when he logs it into his books) and He is NOT allowed to re-sell it in California!
The exception is a consignment sale at a gun shop - that does not fall under the rule of the gun shop logging it in as theirs (a situation that can be better explainedd by a Cal. FFL on Calguns.net).
But, if you have it at a gun shop as a consignment and it doesn't sell you have to re-register it with the state in order to get it back!
So you cannot buy a used colt mustanf from out of state at all...nor from anything but a private person and it has to be face to face and in state.