It just not done on a federal level, each state controls it their own way. For example if a registered gun is sold in New York and it turns up in Florida at a crime scene they will go after the original owner. Responsibility for securing a weapon is put upon the licensed owner. If a child finds a gun around the house and accidentally shoots someone, the owner of the gun will face charges even though he/she may have been out of the house at the time.
It is against State laws to sell a firearm to a unlicensed individual. so no one is getting a free ride on this issue. Just some thing's the federal Goverment should stay out of, let the states handle it. They usually provide heavier fines and punishment anyway.
Yeah each State makes their own laws. New York is an odd example to use as they have some stict gun laws, where are some other States, have no laws what so ever regarding the private sale of a gun from one person to another. So an investigation that traces a gun back to the original owner in Vermont will reveal that the owner sold it to some dude he met in a bar. The sale is perfectly legal and any liability of the original owner ends there. There is no requirment of proof that this sale occured nor any paper trail to show who has owned the gun since it was sold by the original owner.
A gun or hundreds of guns make no difference as no one knows anyway.