I think i know what the problem is. The same thing happened to me, i found out that i SUPPLIED THE wrong paypal address when i sold the item.
So its most likely that the payment is unclaimed or has gone to the wrong paypal address. Paypal told me to send this instructions to the buyer:
Here's what you need to do to cancel an 'Unclaimed' transaction:
1. Log in to your account at Link hidden: Login to view
2. Click the 'History' tab.
3. Select the relevant dates in the Date box and click 'Search' .
4. Click 'Cancel' in the Action column of the unclaimed
(You may only cancel a payment that has a status of 'Unclaimed' and a
'Cancel' button in the Action column).
5. Click 'Cancel Payment'.
6. The funds from the cancelled payment will be returned to either
sender's credit card or the balance of their PayPal account, depending
the funding source used for the original payment. A credit to a credit
may take up to 30 days to process, depending on the card issuer.
When they have done that just tell them to pay again using another method
it worked for me