An unauthorised transaction means that an account holders Paypal account was hacked and used without their consent by your ?buyer. If Paypal check IP addresses etc and find in favour of the account holder then of course they are reimbursed.
However you would not lose out either as long as you met ALL the requirements of seller protection, sadly if the buyer collected the phone then you won't have any seller protection. Why would you not state cash on collection only?
Advice is voluntary. Kudos / Solution appreciated.