For all future manufacture of firearms, there should be indelible (implanted computer chips, perhaps) identification built in to each weapon. Rather in the way we mark our pets with chips to trace them if they're lost; or put VINs in our automobiles. This serial number should also be set into all the parts of the weapon so that no one can construct a firearm from bits and pieces with different numbers to confuse the identification process.
That way, there would be no need to ask the anti-registry private gun seller to do a thing. If his weapon is used to kill someone -- and the shooter was not in the records as owner -- the person who sold that gun (who didn't run a background check and discover that the buyer was a criminal or maniac bent on self destruction) in the database as the lawful owner. And charged with the crime.
Also, if anyone tries to remove or change the indelible or implanted ID, alarms will go off. The owner will face charges.
This idea probably needs some work. Let's think about it some.
Imagine the stamping and etching of all the parts that go into a 1911 or HK or AR-15. Computer chips and alarms built into our guns?
__________________ Gen 1 Glock 17 • Glock 17 RTF2 • Gen 2 19 • Glock 22 Gen4
Glock 23 RTF2 • Glock 27 Gen4 • Ruger Mark I
Ruger Mark III 22/45 • Advantage Arms LE 19/23 Kit 1896 Tula Mosin