By "a returns request" do you mean a case undereBay's money back guarantee? This is not the same as an ordinary return request.)
If it's a money back guarantee case, eBay ought to support you. You've correctly listed it as for parts or not working, and haven't made the usual mistake of claiming that parts of it still work (which allows buyers to dispute this).
EBay's own definition of "for parts or not working" also supports you: An item that does not function as intended or is not fully operational. This includes items that are defective in ways that render them difficult to use, items that require service or repair, or items missing essential components.
It's hard to see how eBay could have found in the buyer's favour if they had read the listing, and their own rules. Another seller posted an almost identical question a week or so ago. EBay sometimes seems to have reached the point of almost automatically supporting buyers.
This is one of the very few situations in which I think it would be worth contesting the case. This should at least prompt eBay to actually read the listing properly, and hopefully reject the buyer's case. It's always a gamble, if you win, it will entitle you to have the neg removed, if it isn't removed automatically. If you lose, you won't get any return of fees and eBay will add an account default to the neg, but this is probably a risk worth taking.
The "parts or not working" condition was supposed to prevent buyers from bidding for non-working items, and then claiming refunds when they didn't work! Unfortunately it doesn't appear to have succeeded.
