I definitely would have said something to the potential purchaser. I would have pulled him/her aside and given them my opinion. It happened to me once before, I was walking into a large popular hardware store that has a car repair shop. I happened to be walking though the entrance to the repair shop when I overheard a service tech explaining to a women who had 3 kids with her, that her car was not road worthy and they couldn't let her leave with it. I stopped and listened for a few minutes. The guy was quoting her $2000 for brakes, tie rod ends, oil change and tune up. She was trying to get her husband on the phone but wasn't having any luck. The guy was pressuring her like crazy. I walked over, asked her if I could speak with her, and pulled her aside. I told her that she should just take the car to my mechanic and he would tell her exactly what needs to be done. I called my mechanic, told him what was going on, and he took her right away. Turns out, yes, she needed brakes, no tie rods. She drove out of there with with new brakes, tune up, oil change a few other minor things....$536.