I was told there would be no sales reporting when I bought a revolver and a .410 shotgun in one visit to the local gunshop. Two handguns, yes, because that was a pattern that met the profile of buying-to-resale engaging-in-business-without-license. My son bought two identical Yugo M70AB2 semi-auto miitary rifles, one for himself, one as a father's day gift to me, from a different store and was told the dealer would have to report the sale especially if he showed up again that month. We are on the TN/VA border and the two states and the feds both have programs dealing with illegal interstate trafficking so YMMV.
The federal law is
18 USC 923 (g)(3)(A); the ATF regulations implimenting the federal law are
27 CFR § 478.126a but in a time or place with an illegal trafficking problem, local requirements may be more than federal law, especially if the state has its own regulations for firearms dealers in addition to federal law. CA, AZ, NM and TX on the Mexican border are exceptional conditions.
"I had heard people say the ATF knocks on your door and questions you ..."
According to the DoJ OIG Report on Operation Fast and Furious, it is standard operating procedure (suspended in Tucson and Phoenix during OFF to allow guns across the border) that, if a dealer alerts ATF to a buyer who matches straw purchaser or buyer for resale as an unlicensed dealer or any other trafficking profile, ATF may elect to follow the purchaser and if the purchaser drops the gun off at an address other than the one on the 4473, they may elect to do a "knock and talk". They may even do a traffic stop and seizure based on "reasonable suspicion".
A Review of ATF’s Operation Fast and Furious and Related Matters DoJ OIG Sep 2012, p. 37:
A “knock and talk” occurs where an agent has reasonable suspicion to believe an individual is a straw purchaser engaged in firearms trafficking and approaches the individual to ask about the firearms that were purchased.
It really takes a lot of shady and suspicious activity to show up and stay on state or federal radar as a straw buyer or unlicensed dealer. Someone honestly matching the profile of hunter, target shooter, gun collector will fall off the screen even though they may get a blip for something like buying two guns of same type, same week.
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[Caveat: Legal advice on the interweb is worth every penny you pay for it. My not so humble opinion is presented solely for its entertainment value, if any.]