Ammo will be the new money.
NEVER, EVER, EVER BARTER YOUR AMMUNITION!
A combat soldier downrange carries a basic load of ammo (mine in Iraq was 200 rounds of 5.56 ball/tracer mix). If you start running low, no problem - First Sergeant in the gun truck had more, just hook up with him and restock.
Well, S just HTF, and guess what? Top ain't there to refill your vest pockets with full magazines. In fact, NO one is. What you have is all you got. And you're going to trade it away? Really? Seriously?
And think for a second - just who are you bartering with? Remember this: The bullet you trade in the morning will be shot back at you in the night. It makes utterly no sense to resupply the zombies from your finite dwindling stash, especially since you've just broadcast to them that you have stuff they want and are armed to defend it; they and their boyfriends will be back that night to take it all, and you.
If you want to barter, then barter skills. If someone's bugging out because they didn't have the brains God gave a rat to prepare for the event, offer to prep their car/truck/moped, and speed them on their way. A good neighbor has something that piques your fancy? Offer a service in exchange, and you'll both benefit. Someone you trust gets hurt, or sick? Have a person in your family or mutual assistance group who's skilled as a medic administer aid to them.
Useful skills and services are things that people will remember more kindly for longer. Barter THOSE, and NOT your ammunition.