Despite an apocalypse, despite disease, war, famine, or widespread disaster, trade and bartering often continues. Trading something you have excess of or that has no use to you for something you have none of is a long standing human tradition. While killing someone over an item is still happens after the Rising, most people would rather trade or barter for items than kill for them.
Year of the Zombie: Traders is a 49 page book detailing trading and bartering in the Post Apocalyptic World, and those people who scrape a living as traders.