The gun store owner who sold the Virginia Tech shooter his weapon says he has a gut instinct about who should and shouldn't get a gun

The gun store owner who sold the gun that was used in the 2007 Virginia Tech shooting says his first line of defense against selling to dangerous people is a simple gut check.

"The first thing that happens is we size the person up," John Markell told the New York Time's Michael Barbaro. "Do you have any idea how many … sales we've lost because we refused to sell the gun? Something didn't smell right. We get lots of mad people. I mean, we've seen some really squirrely people."

Markell's store Roanoke Firearms, located in Roanoke, Virginia, sells a wider range of guns than other hunting-centric stores in the area. Markell estimates he turns away someone weekly or monthly because of a "feeling."

"It's ridiculous," Markell said in an interview that aired on the Time's podcast "The Daily." "It's probably cost me $20,000 of what we've turned down."

According to Markell, employees judged Seung-Hui Cho - who killed 32 in a shooting spree on Virginia Tech's campus - to be a "typical college student" when he came in to buy a gun.

In Virginia, people also need to go through a state and national background test before purchasing a firearm. Cho had no prior criminal record and passed both background tests.

Markell says that he doesn't feel guilty about selling the gun to Cho, and that someone who was suspicious of the student's mental state should have reported him to the authorities. If someone had reported their suspicions, Markell says, Cho would have been placed on the "do not sell" list.

In the US, any gun seller can deny any customer based purely on instincts.