I usually assume, unless it's otherwise stated, that my Apocalypse Worlds have three currencies: bullets, food (including clean water), and gasoline. The three things everybody could always use more of.
But a lot of the time, I like to make it actual barter, too. "You want me to fix your bikes? Dude named Rolfball's got a box I need. Metal, about yay big, some funny stickers with symbols you wouldn't recognize on it. Go get it from him, I don't care how, but be careful with it when bringing it back. And I'll make all your bikes better than before, even."