For item not as described you have to fully refund and pay for the postage of the returned item. You do not fully refund until the item is back with you.
If the buyer has agreed to pay for the return postage cost, that's is between you and the buyer, nothing can be enforced and they can repay you by cheque, bank transfer or a PayPal Friends and Family Payment or you can send them a PayPal invoice.
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.
@10sully30002010