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U.S. President Barack Obama delivers a statement on executive actions to reduce gun violence on January 5, 2016 at the White House in Washington, DC. - 

President Obama said more people who sell guns outside retail stores should be licensed, and they would then be obligated to conduct background checks on buyers.

“Anybody in the business of selling firearm must get a license and conduct background checks or be subject to criminal prosecutions,” President Obama said in a news conference.

Distinguishing between a professional dealer and a mere hobby gun seller is difficult, according to Jens Ludwig, a gun policy researcher at the University of Chicago’s Crime Lab.

“Because the original Gun Control Act was written in an ambiguous way,” he said.

That's the Gun Control Act of 1968. It requires that people who are in the business of selling a firearm get a license to do so. But hobbyists, like folks selling old rifles to upgrade to new ones, needn’t get a license.

“They didn't want to make a hobbyist go through a bunch of paperwork to sell one gun,” Ludwig said.

But in the years since, Ludwig said a gray area has emerged. It consists of people who may not have a storefront but who regularly sell firearms at gun shows or online. 

He has estimated that 30 to 40 percent of U.S. gun transactions do not involve a federally licensed firearm dealer. Some contest that range, but Ludwig said it's still his best estimate. 

In any case, President Obama's executive action calls for greater scrutiny of dealers trying to pass as hobbyists. Their business cards or the frequency of their sales might tip their hand. 

Law professor and gun control skeptic Eugene Volokh said calling out those factors isn’t really a game-changer.

“Those are factors that are already being considered by courts,” he said.

Still, Cassandra Crifasi with the Center for Gun Policy and Research at Johns Hopkins University argued that gun sellers who flout the law are seldom prosecuted.

“It's far more likely that people will prosecute drug traffickers than gun traffickers who have violated laws,” she said.

Crifasi said even a little more clarity on what constitutes a gun dealer could help. The more gun dealers who are licensed, the more buyers who will get background checks.

Follow Annie Baxter at @anniebaxter123