There are always non-paying buyers. There's no point in leaving feedback saying they didn't pay. They can ask eBay to remove it and you will get a warning for feebback policy violation.
When a buyer doesn't pay you need to open an unpaid item case. A case can be opened any time between 2 and 32 days after the listing ends, and closed after four days if they haven't paid.
Your 10% final value fee (but no other fees) will be credited back to your account and you can safely relist the item, or make a second chance offer. The buyer will receive an unpaid item strike from eBay. Neither party can leave feedback. Any already left can be removed. For details, see: http://pages.ebay.co.uk/help/sell/unpaid-items.html
