If the sum of all the forces acting on a body is zero then it is not necessarily in equilibrium. In fact, it will be in translatory equilibrium but may not be in rotational equilibrium. Suppose two equal and opposite forces having their line of actions at a certain distance apart act on a body, then the body will remain stationary at the point but will have rotation due to the torque produced.
If the sum of all the forces acting on a particle is zero then it is necessarily in equilibrium because there will not be a distance between antiparallel forces.
