Well, while the buyer's response strikes me as a bit irrational, the fact is that there are a fair number of buyers on this site who put enormous store on having the original manual, and won't purchase equipment without it. And the simple, hard truth is that you did not accurately describe what you were selling. The honorable thing to do, it seems to me, is to offer a full refund, including shipping, to be paid when the buyer has returned the equipment to you in good condition (I would not refund the money until the equipment has been been returned).
Just one person's opinion.
Just one person's opinion.