Does eBay show the payment as pending? If yes, then you have the incorrect email address on the listing. To check go to your sold item in your My eBay and where it says action click the drop down box and click relist similar, then scroll down to the email receiving address for payment, this will show what email address the payment has gone to.
If the email address is valid and you can access it just add it to your PayPal account via the Profile Tab, your payment will then appear after you have confirmed this email address via the email link received from PayPal. If you cannot add it to PayPal then you need to ask your buyer to cancel the payment at their end they should have the option to do so adjacent the transaction in PayPal, then send them a Request for Payment from your PayPal account detailing the eBay item number and that the request is for goods.
Apparently the eBay app automatically lists your item with the oldest e-mail eBay have on file, which is leading a lot of people to this problem. So if you're using the eBay app be sure to check the e-mail on your listings!