The outside markets should be
on the perimeter, I think, because this way they could be accessed by both city dwellers and outsiders without them crossing each other.
The main safety question is that the city it's a Rash-free zone, where non-immunes can wander freely, while anyone from the outside can potentially be infected, except the ones known to be immunes.
I have a clear idea of how these markets layout could work, and will try to post an illustration tomorrow.
Now, about defending from other humans... Human nature is such that it's probable that occasional robbery or assault could happen, considering the scarcity.
But so far we saw evidence of a strong social order among the survivors (following the Nordics societies actual practice in that matter). Moreover, any bandits acting (or living) in the Silent World would face the inherent risks, which would reduce their number in time.
So I'd expect bandits to exist but in a limited form and acting only on isolated areas. Lone travelers or small groups crossing the Silent World between settlements would have to worry with that additional risk.
I can see a potential Wild West scenario here... but with monsters! A nice idea for a story, don't you think? "The Good, the Bad and the Troll" or "Once upon a time in the Silent World" could be good names...