Im really sorry for u i think its ur fault believing someone.Anyway u should file a complaint to police.
In my biased opinion, PayPal has a reputation of screwing over both the consumer and the seller. I have been screwed as both a buyer (product not delivered) and as a seller (buyer did a chargeback after I shipped the item). As hard as this may be for you, my best recommendation would be to let this $400 go so you get peace of mind and presence of mind. Focus on building your next venture and on making the money back. If you get this $400 back, then that is great, if not, no worries.