Under the
new Facebook "No gun ads" policy, there’s nothing to stop you from posting a picture and details about a gun you own and inviting inquiries about it. If someone contacts you asking how much you want for it, just tell them Facebook policy prohibits using their platform to solicit gun transfers, but if they’re interested in more information about your gun -- for educational purposes only -- they can contact you via email.
If, after establishing a relationship separate from Facebook, you wish to change your mind and consider selling to the person, do what you’d normally do to make sure you’re not being set up by a stranger. And if it turns out that person was a gun-grabber troll, by that time you ought to have validated enough personal information to release it and “out” them as the punk snitch they are, complete with email address, phone number and more. In any case,you won't have transacted any business in violation of Facebook rules.
As a matter of fact, those of us without guns to sell could still set nets up for gunsnitches to blunder in to, as per this example: