Do you mean that you're in AU but the item location is in the US, and you want to list it for sale on the AU site? You just need to change the item location when listing it - at the very end of the postage details section is the item location details, with a link to change the location.
Otherwise, it's simply a matter of being logged into the site you want to list on, but keep in mind other sites have minimum requirements for OS sellers, different rules and policies etc.
