You cannot send anything through eBay if the first refund went through an eBay case, that makes the transaction finalised.
The buyer can pay you now through a bank transfer , cheque or PayPal Friends and Family payment, or you can send them a PayPal invoice. Whichever suits you both best.
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.
@jr2956