Go to Ebay's help function

https://www.ebay.com.au/help/selling/getting-paid/resolving-unpaid-items?id=4137

It's very simple these days, pull up the sale, choose cancel order, tick buyer did not pay,

the buyer gets a strike, you get the fees back.  

On no account leave feedback.

And to add, make sure you put them on your blocker buyers list so they don't pull the same stunt again on you