Otherwise arms dealers have a federal law that protects them in gun/ammo sales from the acts of their buyers.
That said notwithstanding federal protections arms dealers can be liable for the acts of their buyers if the seller knew or should have known the buyer was making the purchase to commit a crime.
Here FB knew or should have known they were being paid to publish an unlawful ad (at least factually FB knew the filters were applied, reviewed these ads and approved them for publication in violation of the law).
I’m not sure why you think preventing FB from knowingly publishing illegal ads is on par with anything else you are suggesting (ie “going after gun sellers”). Gun sales and ads are both legal...until they are not.
Why do you think it’s right to go after the ad creator only and allow FB to continue to profit on and facilitate the illegal acts themselves?