Use the link below to set up the "unpaid item assistant" then after 4 days of non-payment the sale is cancelled, you can also set it to automatically relist the item as well.

I take it you have your "buyer blocks" in place as well.

https://www.ebay.com.au/help/selling/getting-paid/resolving-unpaid-items-buyers?id=4137&st=12&pos=2&...