I think one of the most important things is to use the same email address to open the Paypal account as was used with the payment or add it as an extra address if already opened one with a different address, as long as the card details and the email address are the same the transaction should appear in Paypal and a dispute can be opened as normal.

If that does not prove to be the case then the buyer can ask their card provider to do a chargeback.