Well, did you check the build date on the car? Maybe it was built on a friday before a holiday or something. J/K
But for real, maybe the people had no clue it was in an accident, or whatever. I'm just giving the benefit of the doubt. Honestly, if my mom, sister or even my wife had a car like you described, they would never ever think there was anything wrong with it. Non car people don't know what to look for after a car has been repaired. Maybe they bought it that way and didn't know any better because the car fax was clean. You said they looked surprised when you offered them 6k under. I donno. I always give people too much credit I guess. I'd still be pissed off too, but I'm saying, maybe its not really their fault.
On the other hand...if they did know....then fuck em'. Shame on them for trying to pass that off as mint. I would also like to see pics of the car. What a waste of time. So disappointing.