It is a good practice for both the buyer and seller to do a purchase and sales agreement with the SN and even a copy of the permit (the seller don't need a permit). This will protect the seller if the buyer (or someone he/she has since sold it to) turns "rogue" and use the firearm in a crime and the last trace ends at the seller although the firearm was legally sold at the time. It also protect the buyer from the seller from going "rogue" claiming the firearm was stolen (for example).