"We'll have to agree to disagree there. Ammo will make excellent barter material, especially if the other person has a weapon of one of those calibers which aren't common."
I have 3000+ rounds of both 9mm and 22LR. I'm a decent shot, but if I have to defend with that many rounds, surely I'll be dead somewhere along the way. I have extras for barter, but maybe Old Sarge's point is that you had better know and trust who you're supplying with ammo. For me, it will only be someone in my close group or neighborhood watch.
My point is simply this: the bullets you trade in the morning will be shot back at you in the night. There is no logic in arming the useless eaters to whom you’ve now revealed that you a cache of stuff.
If some mall ninja bought an exotic weapon and failed to plan for ammo needs, then the fault is his, not yours. Never barter ammo; the life you save will be your own.