Hmmm!! This is an oldie on Ebay where the buyer switched their damaged item for the good one they have received, and then claim it arrived broken/damaged.

Unfortunately ebay will normally side with the buyer on these kind of claims as you can see.

I don't have too much for you at the moment except to suggest you get the Australian Federal Police Fraud Squad involved now.

This can be done online via the link below.

The ACORN

Gather all the evidence you believe supports your assertion that the buyer has committed online fraud and make a report.

Once submitted you will get a report registration number.

Pass that report number to ebay in support of an appeal to reverse their initial desision.

You might also pass it to your buyer so they know they are under formal investigation for online fraud.

It's very important to remain calm and profesional as you deal with this.

Especially don't give the buyer any ammunition to come back at you with.

Just be factual and non-accusatory.

Does the item you sent have a serial number at all? 

If so, was that serial number clearly displayed on your listing?