If the buyer has paid using PayPal and the payment doesn't show in your PayPal account history, most likely the payment was sent to an email address which is not linked to your PayPal account.
You may have misspelled your PayPal email address in the payments section of your eBay listing or you may have used an old email address. To check this, click on the "Relist" link for that item and look in the payments section for the PayPal email address.
If you have access to that email address, login to your PayPal account and "Add" the email address to your account. The payment will show up right after you confirm the email address.
Otherwise, you will need to ask the buyer to cancel the payment and resend to your correct email address. Send the buyer a PayPal invoice (Request Money) from your PayPal account to make it easier for the buyer to pay.
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