I'm not entirely sure how it would be possible to make any secure conclusions about the provenance of a gun that was used in a crime. As far as I know, it is not legal to keep a registry of serial numbers and who they were originally sold to. Moreover, in most states, you can sell firearms privately, without any background checks or paperwork at all. I have only purchased one of my firearms in an actual gun shop - the half dozen others are inherited, or purchased from friends, family, or randos on Craigslist or Uncle Henry's. Good luck tracking back the chain of custody on some of those...