Because it's not something a private seller can do. The alleged "gun show loophole" is just folks like you selling what they've got ... do YOU want to be legally obligated to perform a NICS check (you have any idea how to do one?) just because you're selling an old bolt-action .22LR to someone whom you have no reason to suspect?
I sold a Glock 19 to a buddy. Should I _REALLY_ have to run a friggin' background check on someone I know?
Absolutely not! But, what about selling it to someone you do not know...selling it to a felon, an illegal alien for instance?
Thanks! It was the private sale part that I was missing.