回复：紧急求助，有人要告我

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.