As foxettes says, some sellers do not mark items as posted, others mark them as posted well before they are, so while it can be a general indication of the progress of a transaction, it's not gospel. 

The expected delivery date displayed on listings is an estimate from eBay, and while it takes into account the seller's handling time and average delivery time from AP, it's not a promise by the seller or Australia Post (or whatever their chosen delivery service happens to be), and again is ok as a general indication, but fluctuating delivery times should be taken into account, especially if you live in a more remote area. 

If you are concerned, the best thing to do is send a polite enquiry to the seller.