Depends on your state but I'm always a fan of CYA=cover your a$$ so bill of sale at a minimum. Some states require a permit for private transactions, some don't require anything, some require a NICS/State check, etc. Where are you?
"Regardless of whether justified of not, you will feel sad about killing another human being. It is better to be sad than to be room temperature." - Joe B Fricks.
01 FFL, GOFFA, NRA, GOA, IAVA, VFW, ABATE of Iowa D15.