Note that your force grows unboundedly in N, so close to zero you have things that are arbitrarily heavy compared to their distance. So what this paradox really is about, is alternating series' that grow with N, and whether we can say that they add up to zero. If we call the force between the first two bodies f12, then the series of internal forces on this system of bodies (using negative to denote vector component towards zero) looks like -f12+f12-f23+f23-f13+f13-f34..., where, again, each new term is bigger than the last. If you split this sum up by interactions, it's (-f12+f12)+(-f23+f23)+(-f13+f13)..., so "obviously" it adds up to zero. But if you split this sum up by bodies, each term is negative (and growing!) so the sum must be negative infinity. The typical physicist solution is to say that open sets aren't physical, and to get the best answer we should take the limit of compact sets.

The same can be said of unit masses at every whole negative number. The arrow that points to the right is at the same place that the additional guest in Hilbert's Hotel goes. Such unintuitiveness is life when infinities/singularities such as the diverging forces acting on your points are involved.