https://about.usps.com/publications/pub300a/pub300a_v04_revision_072019_tech_024.htm
Facebook Marketplace offers no seller protection. You're taking a gamble. If you feel that you were a victim of fraud then you can start with USPS.
Did you use PayPal business or friends/family? If the former, then they offer some protection. You could call your local police if they are more service oriented. You could call police where he is. This is more of a PayPal issue.
https://www.paypal.com/us/webapps/mpp/ua/useragreement-full#seller-protection
Even though you were a victim of fraud, PayPal should cover you. If you tried to avoid PayPal's fees, you've learned a valuable lesson.