So, any sale among private citizens is illegal without an FFL, right?
WRONG! Person-to-person sales are legal except in a very few states and municipalities.
Does this now mean I have illegal guns?
NO! Unless you live in an area where person-to-person transfers aren't legal, you are underage or otherwise prohibited from owning guns, they are perfectly legal. If you are underage, just have an older relative take possession of them until you come of age.
Calm down. It's perfectly legal to inherit or receive a gun as a gift in virtually every state that I can think of. There might be a little paperwork required in the worst states (HI and CA come to mind)
Contrary to popular belief, guns are not tracked or registered except in a few Draconian states. The NICS and 4473 form are not really registration (though they could be used as such in the future, but that's a different story).
Call a local gun shop and ask if person-to-person gun sales are legal in your state. If they are, all the seller should ever need is a bill-of-sale showing when they sold the gun and to whom. If your state requires some form of gun purchaser ID card, or permission slip from the government, make sure they have it. If you really feel the need to CYA, make a photocopy of the buyer's DL or CWP.
I buy guns from individuals all the time and I've even sold a few that way. It's perfectly legal where I live and I've never had a problem. BTW - When a gun shows up at a crime scene or otherwise falls into the hands of law enforcement, they usually wash it through a computer database to see if it has been reported stolen. If they really need to trace it, they call the manufacturer or importer with the serial number. The factory or importer can look up what FFL dealer the gun was shipped to. LE then calls the gunshop and they can look up the original purchaser in their logbook. LE can then attempt to contact the original buyer and inquire about the disposition of the gun. If the original buyer says, "Gee officer, I sold that gun to a guy at a gun show three years ago!" there isn't much LE can say or do about it.
When I worked in a gun store, people would come in once a week and ask to get a gun transferred into someone else's name (usually a relative). Many times they would get indignant when we said that it wasn't really necessary or required. Sometimes we had them fill out a new 4473 form then keep it for the seller's records.
If you tell us what state and municipality you're in, someone here will probably be able to quote chapter and verse from your local statutes.