The email that you receive to say that an item has been posted is automatically generated either when a seller manually marks the item as posted in eBay or pays for postage and prints the postage label online. Depending on the seller's handling time in the listing they may not be able to actually lodge the parcel immediately.

There seems to be no way to accurately estimate delivery times with Aus Post since they closed some distribution centres. For example, I can post a letter in Geelong to another address in Geelong but it has to travel to the Dandenong sorting centre then back to Geelong for delivery. This can take 2-3 days! As the volume of mail increases in the lead up to Christmas delivery times are also likely to extend.

If you have a tracking number you can generally check the tracking online, provided it has had at least once scanning event.