I’d rather cover my ass than to sell to someone who doesn’t seem legit. We are in NY, remember. Not exactly pro-gun.
For selling to someone you know for sure is prohibited from possessing a firearm it goes against 1 8 USC § 922(d). It’s punishable by up to 10 years in prison.
So if that person is a: felon; drug user or addict; aliens who do not possess a green card or not admitted for permanent residence; subject to a restraining order; prior convictions for domestic assault; fugitive from justice; or dishonorably discharged from the military.