This seems like an abusive buyer. It seems that they may have bought it to finish their rush job and then returned it when they were done with it. How long did they have it before they reported it as broken? You should report them at this link: https://www.ebay.com/help/policies/rules-policies-buyers/abusive-buyer-policy?id=4374&mkevt=1&mkcid=...
Reporting them probably won't help you but if there are enough similar reports, it might protect other sellers from them in the future.
You can't prove they are an abusive buyer though. They may have had a problem setting up the unit that you didn't have, and they sincerely thought it was broken. If so, returning it was within their rights.
As for the amount of the refund, you say you don't accept returns. I assume that means eBay had to step in and issue the return label. When they do so, they don't refund the final value fee. I don't think they ever refund the shipping label or the return label. So the payment to you was roughly $375 (sales price) + roughly $20 (buyer paid shipping) - roughly $14 (your shipping label) - roughly $51 (ebay fees). You collected $330 on the deal. The numbers would be different if the buyer paid sales tax but this is close enough for discussion.
When eBay gives the customer their money back it looks roughly like this for you: -$375 (refund to buyer of sale price) - $20 (refund to buyer of shipping price) - $9 (return shipping label eBay bought for you). You owe eBay roughly $404. You don't get the eBay fees back because eBay had to step in with the money back guarantee program.
If you accepted returns for this listing, you would have gotten your final value fee back and you'd ahead by roughly $51. If you had seller paid returns, the seller might have chosen to be slightly more honest, they might have paid the return shipping, and you'd only be out your original shipping cost.
The good news is that you got the item back in saleable condition. Relist it and collect most of the money again. Good luck!
