We don't know why your buyer returned this item, bit if it was under eBay's money back guarantee and they can provide tracking proof of delivery Bay is likely to ignore anything you say and force you to refund them anyway. This has been the experience of many sellers who have posted here to complain. You may even get a defect on your account for disputing the return.
EBay has reportedly told some sellers that it's up to them to sue the buyer if the return was fraudulent. No-one says it's fair, but the system is massively biased in favour of the buyer.
