The thing is most sellers have cottoned on to the trouble to selling things to Australia so most of them don't anymore which leads to prices from amazon marketplace sellers you don't bother with. Adorama is always worth a laugh if you want to see excessive shipping costs.
If/when the marketplace does come it should be exactly the same as the US/U.K. Marketplace in the way it operates. Who signs up to sell on it will be another matter. In theory it's a whole hell of a lot better than other options such as eBay for buyers anyway.