I know nothing about the reason that this could happen, but I think that it is very honorable for you to show sympathy for the buyer. I know that I would feel bad...(but not necessarily guilty)...and would assist the person in getting the repairs done...how much your financial contribution is to resolving the matter is something that you have to decide. If the repairs weren't very expensive, I would probably offer to split the cost...but once again...the cost of repairs to audio gear is something of which I know even less than the cause of the problem. If the repairs were very expensive I might offer a little "sympathy" cash subsidization...but taking full responsibility on your shoulders?...I don't think you need to go that far.
just my 2c
Ellery
just my 2c
Ellery

