Well, the listing does say registered.

Did the seller give you a tracking number? What does it say? When a parcel requires a signature, the tracking says so, at least with AusPost. ("This parcel requires a signature - Why we cannot leave it".... something like that.)

If they don't reply, open a case with the ombudsman. It takes a long time, but it might be worth it.

You could also go to your local post office and talk to them. I assume it is not very far, as you expected to pick up your parcel from there. When my parcel was stolen years ago, it was the delivery manager of my local post office who arranged compensation.