Sure, locking down a town would be fantastic, however, to keep it together and defending it against bandits is a problem. You could defend it, mabye even put up some more advanced defenses if it gets implented, you might even get together a trustworthy group of a dozen guards. But it would be defenseless when they aren't around, and even with them there anyone who enters for trading could be working for bandits or ready to kill to become one. As YouCanRun said, there are no morals in the apocalypse; people will kill for supplies and weapons just for their own survival - some of them might even be nice people, but that doesn't count for anything when they're low on supplies and all they have is a weapon with a load of ammo. To establish something like this requires leadership, usually from clans, but people not within that clan, or people in general, won't care for the leadership. Even 'protected' (No firing?) zones for trading would be useless, a bandit with a CZ 550/DMR would pick off anyone leaving it with their backpack loaded with fresh supplies. On the subject of survivors working for bandits, I've seen it done before. Me and four others were low on supplies, so we had the only 'survivor' in our group befriend two travellers with nice big packs presumably loaded with supplies. We stalked them during the night and killed them and saved ourselves from starvation & dehydration. People will always be ready to kill in DayZ for needed supplies, and a fortified town loaded with supplies and guards armed with military-grade weapons will be a small obstacle for a group of well-armed greedy and/or hungry bandits. I'm a bandit, yes, I've killed people for supplies and I've killed them in self defense, but I would support the idea of a place where we could trade and feel relatively safe.