You find this encounter by searching a post office. You have the option to approach or leave I chose approach where a survivor with a sniper threatens to shoot you. You then have the option to trade, attack, or run. A man then comes out warning that he has a dozen guns on you so don't try to attack. However, by this point you don't even have thee option to attack any more.
The trade options are 12 Ga shotgun ammo or food in exchange for your med kits, first aid stuff, bandages, sutures, antibiotics, etc. The only option left is to leave.
I never attacked so I don't know what you can gain from it. However, apparently if you don't attack you can keep coming back to trade with them, although I don't see the use in it. First aid stuff is much more valuable than shotgun ammo or food.
I approached and then talked trade, then a guy came out and my options were to trade or attack. I pressed attack and even though I had a sniper rifle equipped I got sniped and lost 30 health with my 50 armor defense. No fight screen was initiated, Then my options were attack again or run. It said gun ports on the side of the building opened up and attacking would be suicide so I ran. Don't attack.