Unfortunately, if you sell by auction, you'll need to get used to buyers not paying once in awhile. There are a lot of irresponsible people out there who do not honor their bids. I've found it is least painful for me as the seller to automate everything so I don't need to send any communication or manually do anything to open or close cases. If you go to your "My Ebay" page, and select the "Account" tab and open the "Site Preferences" page, you can scroll down to "Unpaid Item Assistant." There you can set it up to open the case automatically after two days of no payment, and close the case automatically after 4 more days if payment doesn't come. That gives them a total of 6 days to pay, so your item will potentially be tied up that long before you can safely relist for sale. The buyer will get notifications of the need to pay without your doing anything. You'll get notified when the case is closed, and that you either need to ship if the buyer paid, or you got your final value fee credited back and are free to relist the item for sale again. It's frustrating when buyers don't pay, but at least with this in place I don't need to do anything, and the buyer is reminded to pay by Ebay, and not by me. The other good thing about opening a case (either automatically or manually) is that the "buyer" who doesn't pay will get an unpaid item strike against his/her account. I, like a lot of sellers, have my preferences set to refuse bids from people who have two or more unpaid item strikes on their account. That weeds out some habitual non-payers that I do not want to do business with.
As others have said, it is very important to not offer the item for sale to someone else until the transaction with the original "buyer" is complete. That is frustrating, because the back-up bidder is more likely to still want the item soon after auction end, but you don't wan't to risk ending up with two buyers and one item!
