On the 5th day after the sale find the listing in your awaiting payment section of Orders and use the drop down to select "cancel". you will be asked for the reason and pick non-payment.
This will cancel the sale and issues a non-payment strike to the buyer.
Unlike the old system there is no waiting, there is no second chance for the buyer to pay,. the cancellation and strike are instant. If the non-payment option is not available it's because not enough time has passed.
Note: If you have sent an invoice then the clock starts from the time you sent the invoice
https://www.ebay.ca/help/selling/getting-paid/resolving-unpaid-items-buyers?id=4137
Also on that page is instructions on setting up, adjusting or cancelleing the automatic system. There is also some stuff about fee refunds but normally under Managed Payment there are no fees to refund since they are not charged until the buyer pays.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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