If a case is closed it will after 30 days if no action is taken, then you cannot refund through eBay.
You can repay the buyer through a cheque, bank transfer or send them a PayPal Friends and Family payment.
If PayPal is used, email addresses need to be exchanged, this can sometimes be found with the order details >> Show Contact Info. If not and you need to ask the seller for this, make it very, very clear in your eBay messages, as eBay do read these, the reason for these exchanges. Make it clear it's not to be used for an off eBay sale.