Here's some suggested wording if you're not sure how to word it - but I admit that it's a little confrontational in tone. I could make it friendlier, giving the buyer some wriggle room for claiming that they "found" the "posted" item in the back of their car, and they were sure they'd sent it, but it must have slipped behind the seat, etc.,etc., - but what you've posted indicates that this was cold-bloodedly planned by a "buyer" who is a well-seasoned scammer. In that case, a friendly approach would only signal weakness; I would suggest that a tough approach may be your only hope that they would return the item. In any case, were I you, I would be reporting the buyer as threatened in the letler.
Dear [buyer]
This is a letter of demand in respect of the [item], item number [xxx], purchased by you on [date] from me on the online platform, eBay.
The item was sent to you by Australia Post Express Registered parcel. Delivery was recorded [date].
On [date] you advised me that the item was damaged, and filed a dispute through eBay's resolution process, without providing me with any proof of damage (such as photos). I accepted your request to return the item for a refund on [date].
Subject to your further communications, you were issued by a full refund by me before I had received the item. You were instructed to return the item via a tracked method. You failed to provide any evidence of having sent via a tracked method, and upon my requesting that you provide a copy of the postage receipt, you included a poor resolution image of something sent by regular postage. The details of the postage were impossible to see, and upon my further communications to you where I asked for details, you failed to provide those details.
The item has not been received, and there is to date no evidence that you sent this item back to me. It is now taken, ipso facto, that given that the item was not delivered to me, you have failed to return the item and that you have received a refund under false pretences.
Failing to return an item for which you have been given a full refund is fraud (refund theft). You are hereby advised that you have 48 hours to return the item in the condition in which it was sent, by a trackable method, as per the agreement under which you were issued with a full refund. The address to which the item is to be sent is the same address shown at the top of this letter. You are required to provide me with proof of trackable postage, and the item will be inspected upon arrival in the presence of an authorised witness.
Alternatively, you may return the full refunded amount to me rather than returning the item.
If you fail to comply with the above, your details will be provided to the Australian Cybercrime Online Reporting Network (ACORN), and legal action may be taken to recover the funds or the goods, as determined by our legal advisor.
You may contact me on the phone number (above) to discuss this, and to provide evidence to my satisfaction of having posted the item to my email address (also above).
Yours faithfully,
[name]
