Don't accept a paypal payment of any sort for your ring.   No matter how careful you are there is always the possibility of a chargeback through the buyer's bank if they fund the payment with a credit or Visa debit card.   I am sure the rules in the UK are the same as in Australia in regards to this loophole.

Also, unless you have cash in your hand, do not rely on signed receipts, photos of the buyer wearing the ring or any other "proof" that it has been received by the buyer.