Since when is gun registration required in Missouri??? A bill of sale is not required that I know of, but a quick call to your local PD or SO should confirm that. However, I think it
still a good idea to keep a record of who you sell a firearm to. I have always required
a copy of persons ID/drivers license, and keep this in my gun records - CYA.
Another option, and one I use most often, is to sell the gun on consignment with a local
dealer. The dealer will usually charge a fee of approx. 10-15% of the selling price for
handling the transaction.

Oops - I just checked MO. on the NRA-ILA site, and they do require a Permit to Purchase
for handguns, BUT there is no permit, etc. required for rifles & shotguns.
Link: http://www.nraila.org/GunLaws/Default.aspx