Refunds can only be credited to the original payment method.
The buyer will need to contact their bank for assistance.
If their credit card account has been closed/canceled and they have another credit card with the same issuing bank, the bank can usually apply the refund to the other credit card.
If they no longer have an account with the bank, they should issue the buyer a refund check.
I hope this information has been helpful! As this thread does not involve Checkout By Amazon, I'll be moving this thread to the Help For New Sellers Forum.