Learn the hard way. This is the guy in Lubbock who sold mass shooter Seth Ator the AR-15 he used to kill 7 people in Odessa.
He sold an AR-15-style rifle to a mass shooter. Now he wants universal background checks

For years, Marcus Braziel took his customers at their word when they showed up with wads of cash to buy one of the customized AR-15-style rifles he advertised for sale on the internet.

If they said there was no legal reason to prevent them from owning a gun, Braziel said, that was good enough for him. He saw no need for obtaining a license and conducting formal criminal background checks.
But that was before federal agents showed up at his door in Lubbock, Texas, and told him they’d traced one of his weapons to a mass shooter.
“I wanted to vomit,” Braziel recalled in an exclusive interview with CNN. “My life has not been the same since. I will be forever sorry.”
Braziel, 45, pleaded guilty Wednesday in US District Court in Lubbock to felony charges of dealing and manufacturing firearms without a license and filing a false tax return that failed to report income from his gun sales. He faces up to five years in federal prison under a plea deal with prosecutors.