Has this happened to you?

I have been a buyer/seller an Audiogon for about a year-and-a-half now. So far, I have received nothing but positive reviews as a buyer and a seller. I only sell known good gear and even above and beyond the 7/14 day buyback policy I do whatever it takes to make things right with my buyers. However, two months ago I sold a working amp to a buyer. Just recently I got a message saying it hums. So, I suggested the usual troubleshooting tips and was waiting for the results before I decided to buy it back or fix it (if it was in fact even broken). Instead I get my first negative feedback including a statement to the effect that I knowingly sold a defective amp. I disputed the feedback and it is now under review. I sent a subsequent message to the buyer stating I would buy it back or fix it and pay the shipping. If it turned out to be not broken he would incur the cost of the shipping. Sound fair enough? So far I have not heard back from the buyer. Meanwhile my rating has dropped from 100% to 83%. I'm thinking the dispute should be resolved in my favor. I'm sure some of the rest of you have had similar experiences to this? I mean it's not the end of the world I suppose but still.....