It's going to be an uphill battle. eBay aggressively sides with the buyer. I had a similar thing happen where I had delivery confirmation and positive feedback

It's going to be an uphill battle. eBay aggressively sides with the buyer. I had a similar thing happen where I had delivery confirmation and positive feedback from the buyers but they requested a refund from eBay about a month later. Mine was only for $400 so I just let it go.