'No returns' is for change of mind.

If the item is faulty, the buyer is entitled to a refund. Did you test it before you sold it?

Try working it out with the buyer. If they open a INAD case, you will undoubtedly lose. If you don't resolve the issue, eBay will probably refund the buyer from your funds, tell to keep the item and give you a defect for 'Cases closed without seller resolution'. You will be in trouble if that hits 2% of transactions. So 1 in 50. Which would leave you in serious trouble.