I know this is a fairly old thread, but I can't hold back giving my $.02 worth. I was a Fed LEO for 29 years, stationed at various offices around the US, now retired. I worked with, not for, the FBI and ATF among others and numerous State, County and City agencies. I had to "trace" firearms ownership on various occassions. First time, in my naievete, I asked an associate with ATF for help, he just laughed. We went to the gun store and looked through volumes of 3-ring binders full of forms in random order until we found the record of sale for the gun of interest. No computers, no high level gov't intrigue in Wash DC, none of the TV crap. Subsequent cases involved the same review of binders full of randomly filed forms, but without ATF "help". That's still the way it goes for "tracing" a gun. ATF can, if pressed by a high profile case, check lists of serial numbers by maker to see where a new gun was shipped for initial sale, after that it's check through the 3-ring binders again.
It's good advice to check with local PD on a gun you're buying from an unknown private party, and no matter who you buy from, get a bill of sale or receipt of some kind showing who sold it to you and when. Other than that, don't sweat it.
If the blue helmeted troops do come looking for your guns, it's going to be due to something other than your gun buying history. It will also be time to produce those copies of the receipts you made when you sold them in the parking lot of those gun shows. IF you're still living there instead of in some cave in the mountains.