eBay will apply GST based on where the seller says the item is located, so if they say it's not in Australia, that's all eBay can or will know, and GST will be applied.
Items that ship from China will often arrive with eParcel labels - it's part of the UBI line postage system, (parcels will ship to Aus in bulk, and then get individual Aus post labels when they arrive. This is why some Aus Post tracking results will only show that the label was created for a week or so - the label has been created, but the item still hasn't arrived in Aus yet to be dispatched). UBI usually takes longer than 5 days, but the item could have been on the way to Aus before you ordered it (this is part of what's being called "just in time fulfilment", sales levels for various items can be projected or estimated and items shipped out regularly in advance to a warehouse in Australia, then shipped out via Aus Post.
