It's generally understood or implied that used product are sold 'as is' unless warranty is clearly expressed. Tell him that, and if you do go to court, tell the judge that. I don't want to encourage you to lie, but I certainly would not volunteer the information you reveal here.
Maybe you can offer him a small refund without admitting anything. I believe that's what he's after anyway.
Maybe you can offer him a small refund without admitting anything. I believe that's what he's after anyway.