I just want to add

if you have a buyer who doesn't pay, you do need to go through the process of having the transaction cancelled before offering the item for sale again whether that be via relist, second chance offer etc.

because, until the transaction has been cancelled, the first buyer can still pay, and you will then be legally obliged to provide their item, which will be hard to do if you have entered into a contract to sell it to someone else in the measntime,

as weird as it sounds frustrating and inconvenient that this is, it will be you at fault, and you who could get into trouble,

That's a law thing, not an eBay thing BTW.

so go through the eBay resolution process before trying to sell the item to someone else.