That activity brought him to the attention of the Bureau of Alcohol, Tobacco, Firearms and Explosives, the Tribune’s Jeremy Gorner and Annie Sweeney reported. In 2015, the ATF sent him a letter warning him not to sell more guns until he got a license. He agreed but didn’t stop. In 2017, the agency traced another gun to Caldwell and found he was still posting hundreds of firearms for sale online. Undercover ATF agents arranged to go to his home, where they bought a pistol.