Need advice - buyer claims watch received fake

[roach7]

Hi,
I recently shipped a brand new Tudor BB to a buyer in Australia and now 2 weeks later he's filing a PayPal claim stating that the serial # of the watch received did not match the one on the card. The he claimed that he took it to Rolex and they told him it's a fake. He proceeded to file a PayPal claim and now the funds are being held while the investigation takes place...

Has any seller here encountered such a scam?

Any advice on how I need to proceed with this so PayPal doesn't side with this scammer?

Thanks in advance!