The guy I was dealing with sent me an item that was opened and resealed, and was listed as unopened. I contacted him when I opened the claim, which gave me the option to ask that I keep the item but ask for a partial refund. That's what I did, and he was more than happy to kick back my shipping. I think he'd have asked for more, but the shipping satisfied me.
In your case it sounds even a little more shady than mine (I think my guy it was an honest mistake), and you deserve some money back, especially since yours was opened AND is missing a part. Mine was complete, and I was going to open it anyway, so I didn't want to be a jag about it was all. I wanted something off though because we'd been in a Best Offer negotiation over it and my first offer was declined... Knowing what I know now, I would've wanted it for the lower price though.
He may not like it, or even respond to you, but either way Ebay's probably going to side with you on it. Take some pics of it for evidence.
eBay did side with me - sorry if my post wasn't clear on that point - the only problem was that all they were willing to do was send me a shipping label to send it back to the seller for a full refund.
I even selected the option to chose to keep item/partial refund but they (eBay) ignored it.
If the seller coped to the fact that it was already opened and built and all they did was disassemble it and sell it and were willing to kick back some $$$ - like 25% off or something, I would have been happy with that AND given the seller good feedback.
I did get a eBay customer support survey to fill out - not that I think it will help much - but I plan on giving eBay a less than favorable response on that this weekend.