Both is the correct answer!!!
If you are the buyer its nice to pay after the Paperwork is submitted so you know there is no issue.
If you are the seller you want money before you submit since there will be no argument over funds.
I just had a guy who paid after the DROS was ran and the seller came back in to complain that the buyer shorted him. He wanted us to get involved but that's a civil issue between them and if the buyer passes the check I'm delivering the gun.
Also had buyers complain after all is done that the seller was suppose to give him extra mags/parts/ect.
Do your best to protect yourself.
Not according to BONECUTR and he's an FFLYou can cancel the DROS and call off the sale. Drive home with your gun.
Be happy or pissed. Life goes on.