If there is no tracking number to indicate where it is, you can open a case for Item Not Received.

You can contact the seller, of course, but do not let them dissuade you from opening a case within the timeframe by making promises to send another one etc. And do not close the case if the seller claims they can only refund if you do so.

You have 30 days from the last ETA of the item to open a dispute. After that you're out of luck.

You received feedback from a seller who sells DVDs in the past month, and also in the past. The seller is in Australia and has 100% positive feedback, so I assume he would not ignore you if you write.

Contacting the seller is always the first step when an item is late anyway, and if the seller does not help you can open a case, but I think he would help if it is that seller because he has 100% positive feedback and you have bought a lot from this seller, so he wouldn't want to disappoint you.

Only if it has tracking and the tracking is updating as it should do

As the word suggests, the date inserted by eBay is an estimate (guess) only

And does not take into account things like public holidays/delays etc

Is it 7 days since the first or last  estimated date?

Have you communicated with the seller?

Where was the dvd coming from?

You need to contact your seller.

eBay does not sell anything, only other people selling items on eBay.

And eBay doesn't read here, only other eBay members.