There is a big problem with shipping used tanks to Australia. The problem is that Australia forbids it. Research it. I believe the reason is that they think you are going to send a tank filled with fumes, in other words a bomb.
I learned this the hard way. New Zealand too.
I have gotten around this by declaring the tank as a "motorcycle fairing." I have also had a tank declared as a fairing returned.
New Zealand and Australia also forbid import of used shocks. They are afraid that it will be a gas shock, under pressure, and hence, again, a bomb.
I would shy away from the eBay tank that was just linked. The two fork leg indents are too big, but more importantly, it has a Kreme liner, not a good thing. Maybe the Kreme liner would be okay in Australian -- does Australian gas have ethanol in it?
This recently came up:
https://www.ebay.com/itm/276436954743
A better choice for only a little more cash, except -- it has a 2K paint job. Not sure if that's good or bad -- maybe you can just paint over it?
Depending on how bad you want a tank, you could try to get the buyer to sell it to you off-eBay, and then declare it as a fairing. Buying it on eBay won't work, I think, because eBay uses the item title for the declaration which it automatically generates.