Ebay buyer opened case for a 2 year guarantee sticker on front of camera box?

Saying they can't activate it because it runs out after 30 days. I didn't state in the description about the sticker and have no idea about it. I sold the item as new other - its in excellent condition but has been opened. No warranty has been activated on the camera. They then said they called Nikon and told them the camera takes long time to focus but the superzoom bridge cameras like p600 take ages to focus anyway at long focal lengths. They said it must be faulty so they are using it against me in a case. What do i do?

Say that it was sold in good faith and that you are not a business seller, the manufacturer's guarantee was not part of the item description and that the focus time is a feature of the camera.

Give them their money back.

Discuss with a lawyer. You will need to show your original online post where the camera got sold.