This has a simple answer. Maybe.
You need to function check the weapon, should probably insist on test firing it as well. But also, there is the possibility that the seller, or the person who sold it to them, did not come across the firearm "legally"... Kind of like unprotected sex, you are not only buying it from him, but everyone he has boght it from, etc... and when you didn't see it come across the LGS counter with the factory grease, you don't know when, where, or at who that gun has ever been fired.
SO, be careful, check the serial number, and trust the seller.