its only a pistol registry if you actually have to register them and maintain registration. Ive got several that I brought into the state or got before the private transfer silliness. Even guns that I bought after the transfer law I dont actually have to have in my posession or give any inkling of where they went if LEO comes to the door. Not a registration. Its just like a 4473 in that it only says you bought a gun once.
Sounds like what one of my workmates would say who is a major gun grabber...
OP
I think your being asked for a receipt for one reason only. Its all about tax liability. Your FFL is responsible to collect use tax on dealer to dealer transfers. If your "Private Sale" comes through another then you need to provide a receipt showing them it is in fact a private sale, so the transfer is tax exempt. The state has made your FFL liable for taxes and they would need to be able to show this is a private sale or they would have to pony up. If your seller transferred to your dealer direct then it would be easy for your FFL to show it was a private sale as the firearm came from an individual. If you trade a firearm it is the same type of deal for tax purposes. You shouldn't be liable for the amount of firearms that you trade in against the new price...