Listings on US eBay are automatically in US dollars (this is true for each eBay site - the currency is determined by the site's region), which means if an item sells, payments will be received in USD (so you will need to make sure your PayPal account can accept them). There's also a fee to convert to AUD, so keep that in mind.
You don't need to have an account registered in the US to list on the US (or any other) site, you just need to be signed in to that site. (Also keep in mind that the US site has different policies and a different fee structure than eBay AU).
By listing on an international site, you have to post to that country by default, because even though the item location is Australia, postage to the US is treated as local, so the domestic postage section is where you specify costs to the US, and the international postage section is where you specify P&H to any other countries you want to post to - you won't be able to select Australia Post, as all the services listed on the US site are US postal services, so you need to select one of the generic options (standard international delivery, I think it's called).
