The cops went to all the FFLs in the area and "requested" their 4473s. Every FFL complied. The cops tallied a list of everybody in the area who had purchased such a gun and "requested" them for testing. Every gun owner complied. They did not find the murder weapon, but it was an eye opener. All this was detailed in the Sacramento Bee.
Anybody who has ever purchased a firearm through a FFL is on the radar, as is the gun they purchased. It's already a de facto registration scheme.
My State allowed private sales until last year. That reset my inventory to zero.