If an object is at rest ()at one reference frame, e.g. with respect to you.
I think you would accept that the NET force acting on the object is zero.
Assume there is another observer which is moving with constant velocity with respect to you.
You saw another observer is moving with velocity
Let's back to the previous object, the other observer should find there is no NET force acting on that object.
And that object is moving with with respect to that observer.
From physics point of view: The physics law is the same for all inertia frames, so they all find the same force (or no net force).
An object moving with velocity will be seen as at rest for another observer which is moving at the same velocity .
Assume you site in a train which is moving at constant velocity with respect to ground:
For all observer on the ground, you and other people in the train moving with constant velocity
However, for observer in the same train, all the others are not moving at all ,right? ;)