That sounds a little quest-like, not really bartering. If a "barterer" wants Something, it'd make sense he'll accept that something in trade for anything he's willing to part with in the first place. As in, if the guy is hungry, he doesn't care if you want his extra stack of rubber or extra generator, as long as you bring "enough" food.
Of course, he might Want a stack of water, a pistol and a car, and be willing to trade wildly different (amounts of) things for each item due to their value. Just wait a day and he'll give better price for the water... ?
Wouldn't be a bad idea, at least if it was "integrated" into the world .. one faction is fine with <food>, but requires <ammo>, and some actual lore-wise reasons for the situation.