I had the same problem.  As an old time eBay user who started back when there wasn't an Australian site.

But I didn't go to the lengths that you went to.  I simply closed eBay and went to Facebook marketplace.  Bye.

The nice helpful customer service person on eBay chat told me this...

I have check the details and can see that your register site is of USA (.com) when you register your account on 1998m, you registered on the .com site and now you are uploading the listing on the Australian site (.com.au) that is why it is asking you to sign in on the .com.au site. As you can only have a seller account on AU with the same account you can not have access to both USA and Australia. Here, you will require to create a account on Australian site to be able to list in .com.au and sell.