A: If your question is intended to provide guidance to a prospective advertiser, my suggestion is that you tell the seller that he or she must consult their own attorney to determine if they may sell a firearm without a license.  You do not want to assume liability for legal advice to a seller in a regulated industry.  Even if no license is necessary, that is to say that the person is not engaged in the business of selling firearms, there are other restrictions, e.g., a sale cannot be made to a convicted felon, and some guns may not be sold to out of state purchasers.

With all of that, I do not see any restriction on publishing the advertising so long as the sale is lawful (you would need to check), and the ad is neither false nor misleading.

My recommendation is that under no condition should you tell the advertiser that his or her sale is lawful.