Logically it must be a standard proven by ages bandit's portfolio: gold, booze, tobacco, guns, and a lot of ammo then.

Though, I love the Blast's idea - Electrical energy. It's not just a big solar panel on the roof but also the indispensable knowledge of how to operate and maintain it, how to store the energy, safely recharge different batteries and devices, e.t.c. So in fact that's also a skill set for trade.

By the way. Did you see the "The Book of Eli" movie recently? The guy who could recharge an iPod player is a very reputable person among bandits, and even a happy owner of a junk shop (goods he's traded for charging I believe ) in the middle of the post apocalyptic chaos...