If someone is watching the car, and sees that, they'll be quite tempted to just wait and see how low it goes. Very few cars are so rare, or deals so good, that you can't afford to miss them.
Last time I sold a car (that way), I found the going price for the model and simply undercut it to start with. Gone in a few hours. Perhaps I could have gotten a bit more for it, but it was worth it for the speed with which it all happened.
Some buyers will also watch until you remove the advert, and contact you a couple of weeks later. By then they'll figure the ball is firmly in their court. And it will be.
Of course some cars are much harder to sell than others. It was very easy to sell a RAV, but really hard to get rid of an HSV.
