I feel like I'm going to have to accept the return
Correct. eBay accepts the buyer's word against the seller's for faults.
As for the time delay, eBay gives buyers 30 days in which to report that an item isn't as described.
Sellers can be thankful that PayPal has reduced it's claims time limit to 30 days, the same as eBay. PayPal used to give buyers 180 days to claim that an item was faulty, but this now only applies to non-delivery.)
(Remember that you don't even know how a buyer has paid these days, so you may still be liable to refund for issues reported weeks, or even months after the sale, depending upon what protection the buyer's payment provider provides. Sellers may have to refund buyers without any evidence and without the item even being returned. Buyer protection is a wonderful thing - for buyers!