Has your customer used that card on PayPal before? I had a problem buying with PayPal once. My card was valid, had funds, and I didn't have any credit problems. PayPal wouldn't let me use the card and it got pretty frustrating. After some investigation I realized that the transaction was denied because I had recently moved and forgot to update my address on PayPal. It was right with my bank, but the old one on PayPal, and because it didn't match I guess it got flagged.
I had to use a new card (just for that transaction). My info is now updated and accurate, so I don't have that problem anymore.