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> But, when we are walking, running. We are accelerating.
~~Wrong.When you are walking your moving at a costant speed.~~
> The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else
The book says that because of Netwon's 3rd law which says that forces only come in pairs , for each action there is a reaction.
If two marbles collide then during the contact a force will be exerted on each marble , the one being the reaction of the second force.But those account as internal forces so the total change in momentum of the system is 0.
Hope this helps.

> But, when we are walking, running. We are accelerating.
Wrong.When you are walking you're moving at a costant speed.
> The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else
The book says that because of Netwon's 3rd law which says that forces only come in pairs , for each action there is a reaction.
If two marbles collide then during the contact a force will be exerted on each marble , the one being the reaction of the second force.But those account as internal forces so the total change in momentum of the system is 0.
Hope this helps.