There's a few different reasons a US seller's item will appear on eBay AU but the seller is not actually willing to post to Oz, but AFAIK it's not technically possible to list directly on the Australian site without including a postage option to Aus.
Since the introduction of the Global Shipping Program in the US, when a seller lists an item on the US site, eBay automatically checks if it "qualifies" for the GSP (international shipping services). If they deem the seller's listing qualifies, international shipping options and quotes are automatically set by eBay via the GSP. Sometimes, the seller has no idea their item is being offered internationally until an international buyer sends an enquiry or purchases the item, and they have to manually opt out of this program rather than manually opt in.
Sometimes, it's a problem with eBay's search engine. Items that specifically exclude postage to Australia are not supposed to appear in search results if your location is set to Australia, but ebay's search engine isn't exactly perfect, and will occasionally throw in some "bonus" items, some of which are unavailable to Oz, others are just completely unrelated to the search terms, but that generally isn't the seller's fault or doing. Although I do wonder if it could be caused by the optional (paid) extra available on other eBay sites which, if selected, increases visibility on international sites, but they might still only post to a few select places.
At any rate, yes, in general you can purchase an item, have it sent to a US address, then have it forwarded to afterwards.
