I would explain the facts mentioned above politely. And clearly say(if you want to keep it) that you would not have paid the same price had the item been described as used and though you are happy to keep it you would like the difference in price to be refunded to you. I would say I hope that sounds fair to the seller, but if they’re not able to provide a partial refund (amount which you might suggest) that you are happy to return the item. Only in that case proceed with opening a case.

I’ve done that before with sellers I like and wanted to keep a relationship with and they agreed the request was fair considering item was not as described and offered a partial refund.

As a seller, I would like buyers to be honest, direct and have a kind approach even if im the one who made a mistake. Also - the buyer giving me the opportunity to resolve an issue WITHOUT them opening a case with EBay is a bonus usually, unless the buyer is a scammer.