1. Check local and state laws on both possession and transfer of FA weapons.
2. Make sure the gun in question is registered with the Feds (and the state if required). If the owner can't produce a Form 4, Form 5 or a Form 4467, decline the offer.
Unfortunately, some war vets and folks who inherited FA weapons didn't know about the registration requirements, so the guns are illegal. It is always a good idea to check.