It is already federal law that any gun sale going through a federally licensed firearms dealer (FFL) is required to run a check using the FBI's National Instant Criminal Background Check System (NICs). So initial already yes, updated "if they buy more guns," but still.

Private sales are legal in some states but if you sell to a prohibited possessor you're in deep shit so most people will only do so with a CCW card to show you've been NICs checked and it hasn't been confiscated.