All you can do is grit your teeth and agree to refund them with whatever goodwill you can muster.
I'm afraid that sellers just have to accept the inevitable. EBay's policy is that if they can't determine that an item matched the description, they will find in the buyer's favour and require the seller to refund them. As they never even see the item it's almost a foregone conclusion that they will support the buyer. It's unlikely to make any difference even if the item they return isn't the one you sent them. This is the reality of selling on eBay today.
If they haven't done so already, ask the buyer to open a case under eBay's money back guarantee, through the resolution centre. This won't count against your seller performance rating, provided you click on the option to refund the buyer on return. Be sure to respond within three business days, or the buyer can ask eBay to step in. If eBay enforces a refund you may not even get the item back.
EBay will give you the option to buy and send send the buyer a prepaid return label. When it's returned eBay will process the refund, and you will receive a credit for your final value fee. (PayPal fees are no longer returnable.)
If you dispute the case and lose, which you almost certainly will, you will forfeit the return of your selling fee, and be penalised with a damaging defect on your account. EBay has ways of encouraging sellers not to dispute returns.
