Just about any highly populated area would have some sort of central meeting spot or travel rought in and out, and that's where I'd imagine the trading would take place. But, unless you or someone else is willing to create some sort of 'General Store' of sorts, bartering would be more random.

Ex: someone sees you use a flashlight and happens to need batteries, so he asks you if you have some to spare, so you reply, "what will you trade?" Or someone sees your sidearm and asks if you have some ammo of such and such a caliber... The posibilities are endless.

I don't know if I'd have the time or inclination to set up shop somewhere and announce that I have a hundred extra AA batteries for trade.

If I needed something I would go to where the most people are and announce that I need whatever it is, and if someone said yes, then I'd offer what I've got. All the better if you have a wide selection of stuff to offer, and would make it less likely that you'd even need to barter if you did so.

My $0.02

-Jeff