Can police get involved on ebay?

I sold my old iphone on ebay and just sent it first class buyer says he has not received item demanding a refund which i won't give him and is now threating to get police involved

If you (IF) did not send his item, this is fraud and the police could get involved; but for the most part this is going to be treated as a civil matter thru the courts unless you have done this enough times to have a record with the police for prior fraud.

Paypal will get involved. You should send anything valuable tracked and just have added the cost to your postage rate.
If you did not send it tracked then you are a bit stuffed.
Can you use find my iphone to locate it?
You need to have insured it as part of the postage then they will pay up for none delivery.

Your at fault if no signed proof you sent it

And that boys and girls is why you always use the extra Delivery Confirmation service from your locaL USPS.
If you don't you have no way of knowing if it ever got where it was sent.

Without using the extra Delivery Confirmation (also called USPS Tracking) or Signature Confirmation service from the USPS, you have no proof that the package ever got to the buyer. Also no proof it didn't get to the buyer.

Without that, you may end up having to refund him his money, talk to the USPS and see if they can help you at all.

Always have proof of postage whenever selling things.
also have it signed for.
if it was a paypal transaction paypal will refund them regardless and then you will have to pay paypal
unless you have proof of postage and the delivery was covered insurance wise or
you cancel your bank transactions with paypal and ebay.

then you will start to receive letters from paypal demanding payment and later the request will be passed onto debt collectors by paypal who will send you countless threatning letters demanding payment. You will also no longer be able to use your paypal account until debt is paid

Police would not be interested, as said you should have proof of delivery if not why not?

Also all you can do now is ring the network the phone was last on and get the IMEI blocked permanently.

If you sent it first class, can't it be tracked?

Presumably you sent it Recorded Delivery or at leasst have proof of postage? Take it up with the post office.

My friend police are everywhere, who is to say I'm not a police officer… Or better said a cyber-crime specialist?

The police can get involved if a criminal offence has taken place, but if you are saying you sent the phone but it did not arrive that is a civil matter and therefore police have no powers. It's something you need to take up with Royal Mail.