After 4 days you can cancel and select the reason as 'buyer didn't pay’ (eBay works in 24hr cycles so the option won’t be available until 4 x 24hrs have passed). There’s a link on this eBay page:
https://www.ebay.co.uk/help/selling/getting-paid/resolving-unpaid-items-buyers?id=4137&st=12&pos=1&q...
The non-payer can't leave feedback and gets a non-payment strike against their account, which sticks to it for a year.
Don't be tempted to leave feedback for a non-payer - it can only be positive and will enable them to remove the strike they receive.
Set your Buyer Requirements to block bidders with two or more non-payment strikes in the last 12 months and add them to your Blocked Buyer List with this link:
http://offer.ebay.co.uk/ws/eBayISAPI.dll?BuyerBlockPreferences
You can offer it to the underbidder then, if you wish. However, if you're planning to relist your car, the cheapest and best format to sell a vehicle on is an eBay Classified Ad. This also cuts out the time wasters and non-payers.
A Classified Ad costs £19.99 and there are no final value fees if you sell your item.
Your listing runs for 30 days, during which you can arrange for potential buyers to view your item, you can then end it once paid. You can accept offers too (you don't accept the offer until the buyer has been to view first).
http://pages.ebay.co.uk/help/sell/classified.html
If you decide instead to list in auction format or fixed price Buy It Now it will cost £14.99 for your listing, and that fee is payable even if your item doesn't sell or you end up with a non-payer, and if you sell your item, there are final value fees of 1% of the final transaction (min. £25, max. £45)
