If this was the ring you sold on 17 May it does state "seller does not accept returns" in the listing. However because of the MBG that is only applicable to remorse returns. I am not sure what kind of case the buyer opened with eBay or WHY eBay cannot determine it is past their return timeline as stated in their own policy, but as others stated not accepting returns does not mean no refunds. Depending on how you respond to the case will determine what the outcome will be. If you refuse the return eBay may refund the buyer anyway and allow them to keep the item. If you accept the return and offer a refund once the item is returned depending on the type of case the buyer opened you may be able to partially refund upon receipt of the returned item and refund the purchase price - (shipping cost + eBay fees).
If you decline the return and even if eBay rules in your favor there is always the possibility that the buyer will open a chargeback with their CC company, which usually have longer timelines, and the buyer will most likely be refunded and allowed to keep the item. With a chargeback eBay is removed from the decision making process although they do offer some seller protections against chargebacks. As ckimodog suggested make your life simple and accept returns.
