If the buyer reports that the item isn't as described, then eBay's position is that the buyer is right. eBay haven't seen the item before you posted it nor have they seen what the buyer received, so buyer gets benefit of doubt.
If you have already contacted eBay Customer Services and they've told you that the return is valid then that is that. eBay's money back guarantee does apply for "item not as described" regardless of anything else.
In my view sellers need to be very careful about selling "for parts" items - seller has decided that they cannot or will not test the item ,or sell individual tested parts as working, or be clear which parts do and don't work, yet seller still wants to sell the item for money. But the buyer wants at some (specific) parts to be working for the purchase to be worth it. There is risk because those two sides may not line up. A disappointed buyer normally leads to a return, and a forced return on a "no returns" listing.
In this case buyer finds specific desired parts are fautly so wants to return. Listing wasn't clear (enough) about those parts so buyer assumed what they wanted - those parts working. All logical. If you can't handle that scenario then either only sell tested individual parts or get rid of untested stuff locally where eBay's return policy doesn't apply.
