I'm sure it may depend on the state but when you sell a gun person to person is there anything required by law that you have to do as far as transferring it to the other person?

Over the last several years I have sold some guns to individual persons that I did not know and do not know now so I started thinking about it and with talk of a possible new gun ban, registrations, finger printing etc. is there anything the government could require to prove a gun I used to own was sold?

With the bad economy and all I'm sure a lot of gun owners have sold several of their guns over the years so it seems like a nightmare to accurately track.