Just some info that may be helpful (or maybe not).
Neither eBay nor PayPal have dispute resolution processes for partial refunds (for postage costs) when a delivery takes longer than the ETA. Both are all or nothing disputes, unless the seller voluntarily refunds partial amounts. I don't know if you'll be able to pursue a refund through other avenues, eg directly with the postal service, but usually only the sender can do that.
You've actually touched on quite a few things that are inherently wrong with eBay's ETA dates, and / or the way they are communicated to buyers, especially when international delivery is concerned.
It is almost impossible to estimate delivery timeframes for international postage - yes, there can be averages, or common timeframes, but the frequency of items being delivered outside of those are (generally) so high that the ETA shown on eBay should largely be considered an arbitrary guess that's just barely informed.
The reasons for this are - eBay get their ETAs from the shipping service the seller specifies, they in turn tend to quote how long it takes to reach the destination country, not how long it takes to be delivered to the buyer. The reason the postal services do that is because customs processing and clearance times vary and are not predictable, as is the time it takes for the foreign delivery service to get it to the buyer after it clears customs. For example, Aus Post quote as little as 6 days for international delivery (to the US), via standard postage. The fact that it's only the time they guess it will take to simply land in the US is in the fine print, and eBay ignores said fine print. The number of packages I send to the US that arrive in 6 days is seriously negligible, if I'm lucky they get there in about 10 days, but the average is 2 - 3 weeks, of course if they take longer to clear customs, they can take up to 6 weeks and the thing people should really know is that express postage services doesn't mean express customs clearance - they get processed the same as all other mail, unless coming via courier. I've had economy air mail take 5 days, and express take 4 weeks.
I understand you feel mislead by eBay's ETA, but even in the listing they advise that the dates aren't certainties, and that other factors can effect actual delivery time.
However, this is shown if you click on more info, not by default. the reason they do that is because they don't care if it doesn't arrive when they say it will. Their primary goal is not to give buyers realistic expextations by saying things like "might get there in 7 days, or 21, depending on X number of things", it's to get people to confidently spend money. If they don't get it right, hey it's not their fault, and obviously the first assumption a buyer makes (especially if the delivery time is outside the norm), is that it must be because of something the seller did (or did not do), and so the buyer disputes with the seller and eBay gets to jump in and go "never fear, we'll save you from the bad seller who didn't meet the expectations we created".
With regards to where in Australia a package lands - it rarely goes straight to the airport nearest the buyer, unless the carrier has a distribution centre where it makes logistical sense (and of course unless they live where the carrier's distribution centre is). It might make sense to everyone else to do it that way on first thought, but distribution across Australia is expensive and many companies work the way they do keep the costs down (i.e. a major distribution centre in all states is unrealistic for mant carriers that handle international mail). I order from overseas on a very regular basis, and live in a major metro Adelaide suburb very near the airport - if Aus Post are handling the package, it goes to NSW, and can then take days to reach me, sometimes over a week. If TNT or Toll are handling it, it goes to NSW and then gets trucked over. If DHL are handling it, it comes straight to Adelaide.
