The Craigslist of Guns: Inside Armslist, the online ‘gun show that never ends’

Thomas Caldwell was a veteran in his 60s with a soft physique, oval glasses, no income, and a history of mental illness. “I’ve been schizophrenic all my life, hearing voices,” he once said in a courtroom. He didn’t have a license to sell firearms, but that hadn’t stopped him. In 2015, according to prosecutors, police found a Glock in a Milwaukee drug house and quickly linked it to him. He’d purchased it only the day before.

Months later, an agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF)...