First, let me say sorry you were blind-sided in this way. Ebay looks out for buyer’s interests and sellers are essentially on their own. The most you can hope for is to know what to expect upfront, and prepare oneself for possible fraud by keeping abreast of the policies and procedures governing the site.
Second, are you saying the buyer was refunded twice, once by you with fees withheld, and then again in full measure by eBay? Ebay removed the full amount from your PayPal? Or did they refund just the withheld fees, the balance remaining?
While this is hardly comforting, you are indeed very lucky you got your item back and undamaged. The situation is that a seller can be easily defrauded by a buyer opening a bogus Money Back Guarantee case to ensure a full refund. As a longtime buyer and seller, i am surprised you were not aware of this. You are selling very expensive, high fraud items and i am assuming have not been scammed prior to this.
It is no longer possible to withhold shipping and fees (specifically no restocking fees) from a return unless you meet certain criteria, such as offering free returns or being a Top Rated Seller. Terms of sale also must be spelled out clearly in your listing. No where does it state you will withhold your expenses from a return.
Other more experienced members will hopefully post more helpful advice to you by morning. If your appeal was declined, i cannot think where to go from here except to see if one of the eBay community reps can intervene on your behalf. Meanwhile, was the buyer refunded twice from your PayPal, getting a double refund? If so, surely something can be done to rectify such an egregious error.
