If you really want to get the buyers details I would pull his details from ebay.  That will give you his phone number and suburb.

From there you can try the phone book or the reverse directory.

Even a google search of the name and suburb can prove enlightening.

If the authorities do think it is worth following up they can get the person's details from Australia Post.   He cannot hide from everyone just becaue he has a PO Box.

I honestly think you are giving this whole thing too much time and energy.  Just ignore him and certainly do not send him any emails which may cause him to react badly.

And ring ebay to get the feedback removed....he is certainly not likely to do it no matter how nicely you ask.