It sounds more like a question of economics than ethics. If you sell a unit that has had repairs, you should be very forthright in describing that to any potential buyer. If a broken unit was replaced with new unit, and that new unit has not suffered any working issues, I wouldn't think you'd be obligated to detail any issues you had with a different unit of the same type.
I think there is always value in people relaying their experiences, good or bad, in dealing with manufacturers when things go wrong with their products. I would always wait until I believe the matter is closed, for better or worse, but a factual recount of what happened and your customer satisfaction level is yours to share. Is there going to be a hit to your wallet in doing so - maybe, dunno. Certainly if that is a worry to you, not relaying your story is the safer bet. I don't think you have any obligation to tell a potential buyer about your customer satisfaction level with a manufacturer, just the history and current status of the piece in question.
I think there is always value in people relaying their experiences, good or bad, in dealing with manufacturers when things go wrong with their products. I would always wait until I believe the matter is closed, for better or worse, but a factual recount of what happened and your customer satisfaction level is yours to share. Is there going to be a hit to your wallet in doing so - maybe, dunno. Certainly if that is a worry to you, not relaying your story is the safer bet. I don't think you have any obligation to tell a potential buyer about your customer satisfaction level with a manufacturer, just the history and current status of the piece in question.