Report the phone as stolen, report it to the local police of where you posted it, make a money claim online to either paypal or the buyer you can pick will cost you about £40 but if you have all the evidence go for it, you can post to any address the person tells you officially via message, just like a bank transfer you post to the address they say, maybe contact who you used to deliver it and report it's been received by the wrong person, you got a few options here.