There are different rules for licensees vs non-licensees. Obviously the Feds believe that they have infinite jurisdiction via the commerce clause. The state laws are what they are...
It's questionable if one could sell a rifle face to face within a state without marking it. I wouldn't risk it to be the test case that's for sure, but that does not make it illegal.
If someone believes this is wrong please cite the regulations that are being violated (eg. 27 CFR 478.29 or 27 CFR 478.30) because I could not find anything that would prevent a non-licensee from doing this.