Never relist an unpaid item until you've filed and closed eBay's Unpaid Item Dispute. This process takes minimum 6 full days from the end of listing:
http://pages.ebay.com/help/sell/unpaid-items.html
The alternative is to file eBay's Cancel Transaction Request, but this is only advised when the seller and buyer have a prior, mutual agreement to cancel:
http://pages.ebay.com/help/sell/cancel-transaction-process.html
Either way, if you relist before the the case is closed, you're allowing the original buyer 30 days to pay you... which could be long after the fact you've sold your item to a second, paying buyer:-(
