Do you know what full auto is or are you confusing semi auto with full auto? If it came in a box it is probably a semiauto. Look on the reciever to see what markings are on it. It could be Chinese, Israeli, or US made. It could be a pistol, a rifle, or a short barrel rifle.
If it's a semiauto rifle or pistol the sale would be pretty straightforward with no paperwork. If it is a semiauto short barrel rifle, the sale is a little more complicated if you have the transfer paperwork and are in touch with the family friend.
If it IS full auto, you are looking at prison time for possesing it and the only options are to turn it in to the ATF or stripping it, cutting up the reciever and getting rid of the reciever, then selling the parts as a kit