well done ebay
Just for the record, we're not eBay - just other users like yourself.
From what other sellers have told us, eBay seems to pretty well admit that what you say is true. Typical responses that other sellers tell us they have received from eBay are "Although we understand that you received your item back in a different condition than it was in when you sent it, we unfortunately couldn't determine that this was caused by the buyer" and "As we did not see the item, either as received by the buyer or as returned to the seller, we are not in a position to determine who has the valid case, or take sides".
In every case, eBay's idea of not being able to determine who had the valid case was to find in favour of the buyer, and enforce a refund. The user agreement requires us to accept eBay's decisions, and as their money back guarantee is unregulated there is no right to any independent appeal. The reality for sellers is that eBay has become a very risky place to sell things.