I don’t know where this “Get a Bill of Sale” requirement is coming from. Maybe from members that live in restricted states..
The feds have not been allowed to maintain a registration database or other computerized systems since 1986, thanks to the FOPA. However, law enforcement can contact the National Tracing Center in Martinsburg WV. They have about 50 ATF employees who do gun traces, as well as local staff who microfilmed (and now scan) all the Form 4473s that come in from closed FFLs.
An actual gun trace involves an officer contacting the center and requesting a trace. That officer has to get the manufacturer and model of the gun correct, as well as the serial number. That's harder than it sounds.
Think Browning Hi Powers for a minute. You have the actual Browning marked Hi Power, the FN marked Hi Powers, the Argentine Hi Powers made under license, and the later FM Hi Powers made after that license expired. You also have the Hungarian P9 and P9M "Hi Power" clones, as well as the "counterfeit" FEG P9s that were marked as FN Hi Powers and sold to embargoed middle east countries by FEG. They can be distinguished by the B prefix and 5 digit FEG style serial number, but almost no law enforcement officers know the difference. Then there are the Israeli "Hakeem" Hi Powers that were initially made by FEG and later assembled in Israel from FEG supplied parts.
Worse, imported firearms now have to have new serial numbers applied by the importer due to concerns about duplicate serial numbers.
Think Lugers, P.08s etc, where there were blocks by year with 4 digit serial numbers and an alphanumeric prefix. There are a lot of P.38s with the serial "3891", there are fewer with "e3891" and only a few with a 1944 date and "e3891". If you add in the manufacturer code ("ac", "byf", "CYQ", and "svw") you now have a unique serial number. However, I have never seen an FFL record the manufacturer code, and date as well as the serial and prefix.
Even then, the ATF can't just call Mauser and ask who they shipped a particular P.38 to. If they did, the answer would be along the lines of "the Wehrmacht" and the current German Army probably has no idea where that P.08 got off to. Consequently, unless it's a later import with an importer name stamped on it, the trail will go cold as there is no way to determine the FFL who sold it to get the name off the 4473.
If you are a collector, you need to consider that if we had a registration system, we would not need import marks on all those collectible firearms.
Assuming the officer gets the manufacturer model and serial correct, the tracing agent will call the manufacturer or the importer to see who the wholesaler was they shipped it to. They then call the wholesaler to see what FFL it was shipped to. They then call the FFL who goes through the bound books and 4473s to find the firearm and identify who it was sold to. If the FFL is out of business, the tracing center ends up looking through its microfilm and pdf records to find the firearm.
That's the name - the name of the original purchaser - that will be given to the officer. That's where a bill of sale can be important for two reasons.
If a gun you purchased from an FFL is used in a crime and you were the original purchaser of the firearm, your name will be the name that comes up in a search, 1 day, 1 week, 1 month, 1 year, or 10, 20, or 50 plus years later. Any trace beyond that initial purchase is the responsibility of law enforcement so they will come knocking on your door. Even if you moved, they'll trace you through DMV records, etc and come knocking on your new door.
At that point it's useful to you to be able to show that you sold the firearm to someone else and that you did not own it when the crime was committed.
It's also very useful to law enforcement if you can let them know who you sold it to. Then they can go knock on their door, where again a bill of sale might be useful for them to show it was sold to person #3, etc, etc, etc.
----
Now you can argue that it's none of their business who you sold the gun to. Some folks might be inclined to say that if they have reason to suspect the buyer might have been a prohibited individual. Congratulations, you just made the "person of interest" list. Guess what happens when another firearm you used to own shows up at a crime scene.
It was mentioned above that some buyers don't want a bill of sale and will pay cash. That's fine. The bill of sale isn't for them, it's for the seller, not the buyer. It's a bill of sale, not a receipt.
In my case, if the buyer doesn't want to show a driver's license to show name and address (residency) so I can put that information on a bill of sale, the sale isn't going to proceed. Period. If the buyer changes his mind at that point after being told the sale wasn't happening. Too bad, he's now graduated to the "let's go to the local gun shop and do a NICS check" level.
Why? Because I'm a responsible gun owner.
It's NOT about "restrictive" versus "non restrictive" states. It's NOT about whether the law requires you to identify who you sold a gun too and keep a bill of sale. It's about being responsible about selling guns.
Times have changed and the only way we are going to continue to avoid an eventual gun registration system, and preserve the right to make private sales is if all of us, or substantially all of us, act responsibly to ensure that guns don't get sold to prohibited individuals and that a crime scene gun can be traced to the final purchaser.