Here is a copy of the email I sent the negative buyer.

"Thanks so much for the negative feedback.
I emailed you and apologized that you were having problems getting it to work. I did assure you that it was tested & did work here. I also said that if you were unable to get it to work to please let me know and I would make it right. I would have refunded the money and compensated for your troubles. All you had to do was communicate, but instead you just leave negative feedback. Real professional you are let me tell ya. It's people like you that make eBay an unpleasant experience @ times. I hope for the sake of others, in the future you attempt to work out a problem before leaving your feedback. You would have been surprised what that would have got you had you done so with me. Have a wonderful night.

Chris"