How much was this transaction for? $3000?

You don't have to go through eBay to resolve this. As the saying goes, who died and made them the final arbiter on a dispute. No one did. eBay's decision (in this situation siding with the buyer) is NOT binding on either buyer or seller.

If there's $3,000 at stake, or even less, I would sue that person in their small claims court. Sue them for breach of contract and other causes of action. If this was my sale there's no way I would take a $3K hit. (Note: you can only sue for your damages, meaning the actual cash value of the costume BEFORE the buyer destroyed it.)

Use eBay to show the Judge the terms of the contract. The pictures are evidence of the condition you sent it, the item is evidence of the condition in which it was returned. Get screen captures of all communications, messages, and the page listing.

IMO you have a slam dunk case. If this was on one of those TV Judge shows I can easily envision you winning.

Bonus Tip: You can watch those shows to see how you would present your case. Most small claims cases are the same. You as Plaintiff have the burden of proof. You submit your evidence to the Judge. The defendant (buyer) responds.

Bonus Tip: You should file your case then see if one of those shows wants to broadcast it. Those shows are always looking for interesting cases to air.

For those who think this is far fetched, recall FB disputes. Many sellers who get defamatory negative feedback have asked eBay to remove it. After eBay tells the seller to take a hike, sellers sue the defamer in court.

I did that one time. The buyer had to get a lawyer and work out a resolution with me. He issued a public apology, paid me a few thousand in damages, and had to provide me a letter of apology. (I rather enjoyed the buyer in a rude tone telling me to get sc....d (because he thought nothing would happen) to his having to pay his lawyer and me, and morph into a pantywaist begging me to settle the case.)

By the way, eBay deciding against you is not a binding decision. You are free to sue the buyer any time you want (but usually withing the statute of limitations in his state where the case will be filed.

Bonus Comment: You have to sue him or her where they resides (where you shipped the item). Check online how to go about filing a small claims lawsuit against them. Many states let you handle the case online, up until the court date.

I suspect that once sued that buyer will settle the case with you.

(Note: save the costume. It is evidence.)

Bonus Tip: You can search YouTube, ideally to see if there ever was an eBay case on one of those shows, but otherwise you can watch the hundreds of shows to see how these cases are done.