From Ammoland:

The homeowner, who did not want his name used in this story, is a small businessman – a law-abiding citizen who even ATF acknowledged had done nothing wrong. He went to his front door hesitantly. He left his doorbell camera running to record the encounter.

The older ATF agent told him they were assigned to a task force investigating straw purchases. A straw purchase – a federal felony – occurs when someone buys a firearm on behalf of another person, who is unable to legally purchase a firearm themselves.