what do you do if you ask someone if he is a felon or otherwise prohibited form owning a firearm, he says no, you do the dew, and alter find out form another source he is infact prohibited? purely hypothetical here.
:sign0101:
If you didn't know when you sold it, there's nothing that is going to happen. You have to knowingly sell to a prohibited person to get in trouble.
If I found out he was prohibited? I'd start with talking to him, because he might honestly not know he's prohibited.