Nothing you can do from eBay now but to rely on the buyer's honesty to repay you.
Send them a polite message that you are pleased the item has now arrived, and for the repayment to be made by a cheque, PayPal Friends and Family Payment or a bank transfer. Or, you can send them a PayPal invoice for the 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.
@dymafi
