An avocado is rotten if it's black on the outside and, when pressed with a finger, leaves an indentation mark. However, when an avocado is cut and the insides are either brown or black, this is another telltale sign that an avocado is not good to eat. When buying an avocado, it should be slightly firm when pressed with a finger, and its color should be light to dark green.
After buying avocados, it's important to store them properly to extend their shelf life. Fresh avocados can last between 3 to 4 days if stored on a kitchen counter. Store avocados in the refrigerator to extend their shelf life from 7 to 10 days after they're ripe.