Make sure you're on the ebay site that the seller has listed on. ebay.com or ebay.co.uk etc... if a seller doesnt list that he will post overseas, you might not be able to view their items on ebay.com.au
I have this problem when I use the eBay mobile app
I normally refine the search to include 'worldwide' location and all search results will do this, but when you look at seller's other items, you need to 'refine' the search again to include worldwide!