Go to your selling page, find the transaction and see if the '$' sign is a solid blue. If it is, it's likely that this is just a Paypal glitch. There is a known glitch - paypal sometimes takes 3 days to post a payment.
If the '$' sign isn't solid blue, then make sure you put the correct Paypal address in your listing. In the same line, look for a drop-down list to the right, click on the arrow and choose "Sell similar". Scroll through to the second page to see what your paypal address says, confirm that it's the same, then cancel at the bottom so you don't actually list the item again.
If your paypal address was wrong, the customer will have to cancel his payment and re-issue it to you.
If it was right, he's likely lying - ask him for a transaction ID number and call Paypal to check where it is.
