How do I prevent selling a firearm to a customer whose background check is still pending?

The most reliable way to prevent this is a POS system that automatically places a firearm in a pending/hold status when a 4473 is initiated and a NICS check is in progress. The system should block that specific serial number from being rung up at any register until the background check is resolved — approved, denied, or the three-business-day waiting period expires.