Marking the close means buying or selling an insignificant amount of stock right before the close of trading. Also, manipulating the stock at the close to create a misinterpretation of the day's trading action is illegal as well. Today's last trade at 3:57 et of 100 shares is a good example of this because the trade price is quite lower than the stocks trading range for the day making it seem as if the stock didn't do as well on that day.