eBay Buyer Claims Authentic Item is Fake – Wants 50% Refund, What Should I Do?

I need some advice. I sold an item that I know is 100% authentic, but the buyer is claiming it's a replica and is insisting on keeping it for a 50% refund. This is my first time dealing with a return like this, and I really don’t want to issue a partial refund since:

  1. The item is authentic
  2. I no longer have the item in my possession
  3. This would mean I sold it at a loss

I don’t do a high volume of sales, just about 1-2 items per week, but I still have 100% positive feedback and would like to keep it that way. I know eBay tends to side with buyers in cases like this, but I'd rather not take an unfair hit here.

What are my options? Should I refuse the partial refund and let eBay decide? Or is there a better way to handle this to minimize losses? Any insight from experienced sellers would be greatly appreciated!

Thanks!