Well, sounds as though that seller you encountered on Facebook was looking to lure you into a trap in order to get you a warning - does this seller offer the same stuff you do?
What you should have done was have this person from Facebook initiate messages to YOU about what they may want you to do. By you sending a Custom Offer without any prior conversations initiated by a buyer, this looks to Fiverr as though you are spamming. Certainly, Fiverr would have not known unless this person reported your offer as spam.
I would in the future avoid carrying on conversations on Facebook and just tell the potential buyer to message you FIRST on Fiverr and give them your link. That way, you are safe from getting a warning, unless, of course, you keep messaging them after they haven’t responded.
I am not sure how Fiverr would help you given much of this was done off platform. While most reading this would realize you were not indiscriminately sending offers to strangers here - Fiverr may not see it that way.