I would require it for legal purposes. Private sales are completely legal, but that does not absolve you of liability in any manner whatsoever.
If it's not required by law, then it can't be illegal to not do it.
What liability concerns you post private sale?
If you sold a firearm to someone and have no proof and any due diligence, it could get legally very bloody. The moment you have to pay a lawyer you've already lost.