I have to assume that the unit you're going to be selling is either new (i.e., a replacement unit) or repaired. If it were the latter, I would disclose that fact, but if it were the former, I can see your quandry. I would definitely tell a buyer about the circumstances around the unit if they were to ask about it; whether I would so state in the ad would depend. If the replaced unit had functioned properly for a long time but then broke down from age, and my only complaint was that the manufacturer/distributor would only replace rather than repair the unit, I might not feel compelled to mention it in the ad. On the other hand, if it broke the day after its one year warranty was up and I had doubts about that model's reliability in general, I'd feel differently about not disclosing it in the ad, because the next buyer might have the same problem after a week or so with the new unit.
I had a similar situation many, many years ago back in the late 80s--a well regarded CD player I got was in for repairs within a month or two after I bought it with a skipping laser, and stayed in the repair shop for over 6 months. When it came back, I had replaced it with another manufacturer's player, so I sold it to a friend, and I did disclose the prior problem and the long repair time. Of course, after the sale it worked great for a month, but broke down again with the same problem, right after the warranty had expired so I took it back from my friend and gave him his money back. Ultimately, after I wrote to the manufacturer, the manufacturer was good enough to let me buy a new unit of the replacement model for dealer cost to make up for the bad experience, and after discussing the pros and cons with him I let my friend buy that replacement unit for that price with the undertanding that I'd buy it back from him if it had the same kind of problems within a reasonable peiod after he started using it--fortunately, the unit actually is still in use today.
I guess that the way I think you should look at it is what would you do if the person you were going to sell it to on the A-goN was your friend. Follow that way of dealing and your conscience will be clear and you'll have less problems if something goes wrong. Just my $0.015--inflation's getting worse by the hour.