In the past, I have done a two-part bill of sale.
I, NavyLT, did on Feb 1, 2011 sell one Taurus PT-145, serial number XXXXXX to Joe Citizen and did receive $250 as the purchase price of same gun. Signed: NavyLT. The buyer gets that one as a receipt for his money.
I, Joe Citizen, did on Feb 1, 2011 purchase one Taurus PT-145, serial number XXXXX from Navy LT for the purchase price of $250 and did receive same firearm. Signed: Joe Citizen. I would get that one as a receipt for my gun.
Although not required by law, I would show an ID card for verification of state of residence and age, but no way am I providing a copy of it nor would require a copy from the other party. If you don't trust them enough to buy/sell a gun from them, then why would you trust them with a copy of an ID card AND if I am not willing to give them a copy of my DL, then I do not feel it appropriate to ask one from them.