Some buyers will take a few days to pay, others you'll never hear from again, so what I recommend is a polite message to the buyer a few days after the sale, depending on how long you're prepared to wait, and if no response (I allow around 48 hours for a response), open an unpaid item case (next to the sold item, click More actions to bring up a dropdown menu of options, then click Resolve a problem - this can only be done after 4 days have passed, and the case will automatically close if the buyer pays via PayPal, or you can close it manually after another 4 full days have passed to get a credit on the final value fee).
