I have had a similiar issue before, however in this case I would contact ebay customer care and explain the situation. I personally have to agree with another AK'er that was taking pictures of the internals and serial numbers to prevent from swapping out pieces and declaring the item broken.
I have found that when I had a buyer complain about an item not working, I first contacted them and offered my phone number directly. I then asked them t define what was " not working" in detail so I could address the issue. Then I moved to ask them what they wanted to do, if the problem was something I could address or if it was obvious that it was a lie. When the buyer advised they wanted a refund and shipping returned I advised that the item was documented and recorded as working, all proof would then be submitted to customer care to be reviewed. Needless to say the information was enough to close the case in my favor and postive feedback was left.
I can't say this will always happen, but I believe that the more you take the lead of attempting to address the issue with the buyer, the better the outcome in the eyes of the customer service. I also felt that talking to them directly yeilded a better response verses letting the issue ride out on messages and exchanging of the facts.
I know that Jim on AK has taken an extra step as have I about recording the item functioning, then packing, shipment, tracking and insurance. This is extremely important especially with items that are of high value.
If you like you can PM me and I can try to help you, I'm sorry to hear you're having so many issues
Best of luck
Kevin