Steven W. Clyne, 69, is accused of selling firearms without a dealer license.
A federal grand jury meeting in Boise indicted Clyne on May 11. He made his first appearance Tuesday in U.S. District Court. He was released on his own recognizance.
He is accused of illegally selling firearms between January 2013 and Nov. 12, 2015.
Clyne, who does not have a criminal record in Idaho, according to state online court records, is scheduled to go to trial Aug. 8.
If convicted, he could face up to five years in prison and a fine of up to $250,000.
Federal prosecutors are also seeking the forfeiture of 26 rifles, shotguns and handguns seized from Clyne.
The case was investigated by the Bureau of Alcohol, Tobacco, Firearms and Explosives.