Average velocity will be zero, if the displacement is zero. As we all know that, velocity is the rate of change of displacement.
Consider an object starting from a point A, which moves to a point B. The, it returns to the point A again. Here, the displacement of the body is zero, as the starting point is the ending point. But, the body has taken some time to finish this task, right!!! Therefore, let the time be x.
Though, 0 divided by anything will be zero, right!!! Therefore, the velocity of the body= 0/x =0.
Therefore, in this case, the average velocity of the body in overall will be zero.
Hope this will help you.