Whenever you sell a firearm, it's best for both parties to have a little papertrail.
Take two pieces of paper. On the first write: In consideration of $X cash money received this 21st of July, 2011 from him, I do hereby sell Mr. John Smith of Baltimore, MD my Remington 700 CDL .30-06 rifle serial number 123456. I represent that this firearm is my lawful propery to sell. Signed... Dttomcat, Baltimore, MD.
On the second page, write: In exchange for $x cash money which I have paid to him on this 21st of July, 2011, I am buying from Mr. Dttomcat of Baltimore, MD his Remington 700 CDL .30-06 rifle serial number 123456. I am taking immediate possession of this firearm. Signed... John Smith, Baltimore, MD
You each sign the appropriate paper and each receives the opposite.
If the gun is subsequently found to have been used in a crime in Seattle, Washington last year, Mr. Smith has some plausible evidence that the purchased the gun this year in Baltimore, Maryland from someone who represented the gun as legitimate.
If, on the other hand, next year the police knock on your door saying they found your gun at a crime scene in Newark, New Jersey, you have plausible evidence that you sold it this year to a guy in Baltimore, MD who took possession of it at that time.