Legally it means NOTHING unless a notary present - it's just a piece of paper that makes you feel good: false security is the worst kind.
1. If you think the buyer shouldn't buy the weapon - DO NOT SELL IT
2. If you are paranoid and want a paper trail - get it 4473ed
3. Otherwise - obey the law: if you believe the buyer is legit - shake the hand after taking the cash.
GF is a notary