The incident took place before dawn Saturday outside a shoe store in Atlanta's Little Five Points area.
The group was waiting for the store to open for the day so they could buy the LeBron X Denim on its first day of release.
Police said the man approached the group with a gun in hand and tried to rob them.
One of the men in the group took out his own handgun and fired, said Atlanta police spokesman Carlos Campos.
"A number of witnesses were interviewed and this appears to be self defense," he said.
Campos said the customer was not charged.
Another customer in line, Taylor White, told CNN affiliate WSB-TV that the would-be robber should have thought twice.
"I didn't even expect him to come up here, thinking it was that sweet. Thinking it's that candy land like that," White said. "He wanted to pick-pocket everybody. But people out here, they weren't going for none of that."