If you filled out a 4473 when you bought the gun, and the record of the sale went in some FFL's bound book, there is a paper trail to you. Getting proof in writing that you sold it legally and responsibly is your best protection.
If you can be sure that the gun can't be traced back to you, you are out of the reach of any liability, so all you have to do is follow the law where you live.
Selling it to a stranger out of your area, with no information exchanged could pretty much assure anonymity, but it is probably not legal, and I would have a problem, morally, with that.
Selling it to some guy at a gun show, who shows you a valid ID and CCW, but you write down nothing, is legal in a lot of places, and gives you reasonable assurance that you are not selling to an unqualified person. It doesn't assure absolute anonymity though.
JOIN THE NRA!