I've sold a ton of stuff on FaceBook and used PayPal as the transaction avenue and have had zero problems. I know there's definitely scams out there though, but saying this is 100% a scam is way overblown.
OP - you can be scammed via PayPal, just like anything else. If you meet in person they can just rob you, or give you counterfeit currency. My point is there's some risk involved in every transaction dealing with used goods. If he pays you via PayPal and you login to your PayPal account via your own link (not one from an email PayPal sends) and verify the money is there, then you're safe to ship. I'd document with pictures that the phone is in the box, and ship it with signature required. That's about the best protection you can get. If you can talk him into sending it "friends and family", then he won't get any protections from PayPal and that further reduces the risk of being scammed. If I was the buyer though, I wouldn't go for that because then he doesn't have any protections in the event you try to scam him (not that you would, but speaking generally).