If eBay turned down a case that was genuine, the buyer could sue them for failing to honour their money back guarantee. EBay doesn't give any guarantees to sellers, so the safe and easy answer is to give the benefit of any doubt to the buyer, and make the seller refund them.
Plus of course the fact that providing the buyer with a good experience is eBay's stated priority.
No-one says that forcing sellers into refunds without any evidence is fair, but the user agreement gives eBay the power to reach these decisions and we have agreed to accept them. EBay is unregulated, so there is no right of appeal to any independent appeal. This has become a very risky place to sell things.
