If you are top rated and have at least 30 day returns, I would withhold whatever the value lost is. You can deduct up to 50% of a refund in those cases. Unfortunately if you are not top rated or don't have 30 day returns then the buyer got a free case and you are just out of luck.
eBay does not tend to really care enough to even look into it, if we are being honest. If you are lucky they might use up your one time courtesy, but after you play that card, you are out of luck later on. If you are not eligible to deduct a portion of a refund, then I am sorry but you got ripped off and there is not much to be done about it.
In that case, about the only thing to do is message the buyer and ask them to return your case. If they say no or ignore you, then you can file a police report and send a copy to eBay. They might help you if you go to those extremes.
