What should I do, need advice?

The other day I put an ad on gumtree to swap my phone for an iPhone a guy agreed and came to swap when I got in the phone was scratched and the lock button was broke the chargers were broke and my sim wouldn't work, mine was in perfect condition quite new I rang him constantly for 2 days and got no answer or reply so naturally I believed I had been ripped off and blocked my phone another 2 days past and this guy starts constantly ringing and texting saying he's been to the police with it and its fraud and everything, I got in contact with him and he suddenly blurted out that he had taken a secret picture of me even though I'm under 18 "just incase" surely this isn't ok as I had no idea and it was unconsented to? He is now saying the police have confiscated the phone and he doesn't want it and he wants me to go to the police and get that phone even though I have said i will unblock it and give my consent for him to have it he is saying now that he hasn't given them my name and if i don't pay him or buy him an iPhone he will give them all my details he won't leave me alone and is saying he will take necessary action as he knows my postcode this seems threatening to me as I was honest and have said I will unblock it, I have no money and don't know what to do? Is he right or am I in the right? It is causing me a lot of stress and I can't sleep at night

You are very much in the right here, and he is wrong to try to intimidate you. If I were in your place, I would contact the police and tell them what he has been doing. I highly doubt half of what he told you is true, but the point is, he tried to trample on your rights, and he hasn't stopped trying. Get an authority to help you out, and tell them everything you can remember about his behavior and the whole situation.