First off, cancel the sale as mentioned, using something wrong with buyer address. You can do that by going to the item on your selling page, clicking the drop down on the right and select cancel item.

Add the buyer to your blocked buyer list. Click on this link and add their username https://offer.ebay.com.au/ws/eBayISAPI.dll?BidderBlockLogin&BidderBlockLogin=

Go into your settings and block international buyers. You can do that here https://cgi5.ebay.com.au/ws/eBayISAPI.dll?ExcludeShippingList Make sure all boxes are ticked, except PO boxes.

Next, click this link, https://offer.ebay.com.au/ws/eBayISAPI.dll?BuyerBlockPreferences&BuyerBlockPreferences= and set up buyer requirement blocks. Change the settings to as follows. 

Buyers with unpaid strikes, 2 in 12 months

Buyers in a location I don't post to (this is the one to stop overseas buyers being able to buy)

Buyers with policy violations, change to 4 in 6 months

Buyers who have not gone through phone verification, tick that.

Apply above to active and future listings, tick.

Don't allow blocked buyers to contact me, tick that.

That will give you the highest level of protection. In the interim, get on and cancel that sale. You're in for a nightmare if you don't.

Please come back and let us know how you go.