The price an item sold for and what it cost originally has nothing to do with a not as described case.

Your refund policy applies to what you will do for change of mind, it has nothing to do with a buyer getting a refund if an item is not as described.

As suggested ask for a photo, if it shows a mark then I would offer either to send them a prepaid satchel to return it for a full refund (If you think it is worth the cost of getting it back) or to give them a partial refund and they keep it.

Making a success of selling on ebay is all about customer service, put yourself in the buyer's shoes and think how you would like to be treated.

As for you examining the item carefully, even the most experienced of us miss a small fault now and then, if you are 100% sure you are infallible then you could just say you are not refunding as you are always right!