The bill of sale is not a legal requirement although a good idea to protect both sides of a transaction.

What you have to have to keep everything legal is proof of residency (e.g., CO driver's license) -- BUT his military ID to show current active duty status and a copy of his PCS orders showing he is stationed here in Colorado meets the legal requirement. You (or she) don't even have to keep a copy of his orders, just see them to verify authenticity.