ahmed goes to a gun show to buy. what criteria must he pass in order to buy a gun?
how do we know whether "he's off his meds"? how do we know he has a brain tumor?
how do we know he tried to commit suicide four years ago? how do we know he's not
a front for a terrorist cell?
Background checks are not foolproof. Nobody is suggesting they will prevent everyone who should not get a gun from getting one. It is simply a reasonable impediment to put in place so a known terrorist, criminal, menatlly ill person can not walk in to a gun show and walk out with a weapon with no questions asked.