Message the underbidder and ask if they are interested in the phone as the winner states accidentally bid.

If interested inform them that you will send them a SCO as soon as the unpaid item case is closed.

Exactly 4 days after the end date open an unpaid item case as there is no way someone can accidentally bid in a 2 step process.

Buyer is issued with a strike on their account.

Then close the case 4 days later. Neither party can leave feedback and if underbidder doesn't buy then relist.

Also add the non payer to your BBL and set up blocks on your account to avoid serial non payers.