There's nothing to report. The buyer has done nothing wrong in Ebay's eyes - all they have asked you to do is undervalue the item. This is not an Ebay issue. Ebay will only become involved IF the buyer opens a dispute.

What you could do is contact Aus Post in a private message on their Facebook page. Tell them you sold an item and that you have a bad feeling about the transaction and the buyer and that you think you might be getting scammed. Keep the information simple. Do not tell them the value, but give them the tracking number. Ask them if it is possible to have the item redirected back to you if it hasn't already left the country. it may or may not be possible, but doesn't hurt to ask.