There's an end to this story and some lessons learned.
I'd hoped SeverousSilver would have responded, however as the other party I feel I can share this.
As in the OP, we determined a price, $2200, and I mailed four coins when funds cleared. Unfortunately SeverousSilver advised his street address wrongly, and I sent the package to this wrong address. His amending PM arrived after mailing. I assumed AustPost would return the package as 'unable to deliver'.
The day after delivery, I received a phone call from the AustPost delivery man (since my phone number was on the package) saying 'No such number'.
I advised the corrected address, not knowing it was also incorrect. I'd originally been advised the flat number, and SeverousSilver had amended with the building number. The AustPost driver would probably not have been able to connect the two numbers.
I corresponded with SeverousSilver fairly intensively over the next few days. If you read the OP you'll get the gist of it. His fear was that the coins had been stolen, or taken by the driver.
I felt a liability since: I had advised the driver to redirect the package. I had not kept my receipt to prove Sign on Delivery, and I had neglected to describe 'At your risk' in the transaction (despite my own advice here).
I therefore decided to send SeverousSilver two additional coins, being half the four that had gone missing. Assuming 50% liability.
Both of us were thus a little unhappy at how the trade had worked out.
Then, about a week ago, the package in question was returned to my PO Box.
I contacted SeverousSilver advising such and offered to 1. Receive back the 2 coins I'd sent in settlement of liability, or 2. Receive payment for an additional two coins and mail the original package, or 3. Mail 2 additional coins.
SeverousSilver opted for option 2 - paying for the additional two coins I sent, and when funds were received I mailed the original package to his newly obtained PO Box.
AustPost tracking confirms the delivery, as does SeverousSilver, and I have filed the Signature on Delivery receipt.
In summary:
I suggest traders here include mobile numbers for their packages.
I'd reiterate, no street addresses.
Australia Post's delivery records are not necessarily accurate.
If properly packaged and labelled you'll most likely receive back any undelivered parcel.
There is also the possibility that a thief got cold feet and decided the best option was to put it in the nearest mailbox with 'return - no address' but I think it was likely a comedy of errors that occurred.
Finis.
p.s. and I have learned:
Only PO Boxes.
Keep receipts even after delivery.
Phone numbers on parcel are valuable.
Aust Post drivers are probably honest and show commonsense.