According to the ATF, Marcus' shop and 430 other dealers throughout the country were singled out because they had sold 10 or more guns used in a crime within three years of purchase.The ATF easily can trace a gun from the manufacturer to the first retail seller to the first buyer. Second-hand sales are much more difficult to trace unless the agency knows where to look among the nation's 100,000 gun dealers.
If it's so hard to trace that they need to compile a registry (sorry, a "list"), how were they able to trace at least 4,300 guns to 430 stores without a list?
I believe the mfg's know what deal/person they shipped each gun to and they let the ATF know if they are requested to trace the gun by ATF. Then the dealer knows who he sold it to by the paperwork. Guns get traced when they turn up in a crime or are found abandoned or "dropped' someplace.