I just selected Express Post for an item in NSW.
Looking at the Aust Post website, the express post is guarantee is next business day delivery or a refund on the postage. It will be posted 27th, so should hopefully arrive 28th in VIC.
However I noticed eBay put the estimate as Friday 29th. Does what ebay write, override the Aust Post guarantee??
The guarantee would apply to the sender (you won't get any refund for postage except if you try for a refund from PayPal or ebay under not as described etc). Auspost won't care what the item is or how it was sold, the ebay listing means nothing to them.