Unfortunately you let your emotions get the better of you in your response. Always avoid using accusatory terms like "You did this, You said that..." We (BPers) don't doubt that you are a AFOL, but the person buying your set doesn't care. (usually)
Since you communicated directly through ebay, ebay will see the communication chain.
Before you take the next step call ebay and talk with them. They'll look over the correspondence, the listing and your track record and help make a viable recommendation for you.
Just be prepared for ebay to not rule in your favor. Hopefully this wont be the case, but still...
If it was me I'd approve the return and move on. As others have said, it's the cost of doing business on ebay.