This may be due to an error with the PayPal account, internet connection or maybe the item is no longer available to purchase (i.e. someone purchased it just before the buyer).
We recommend advising the buyer to try again and if the payment is still failing then please get in touch with PayPal to make sure there are no restrictions on your PayPal account.
If you have any questions at all on this, then please feel free to get in touch with support. Our team are always happy to help out in any way they can!