I have to respectfully disagree.
While water is important, I believe that trade items will be what have been trade items historically.
Booze and tobacco.
/johnny
You left out something older johnny, but then this is a family forum. ;-)
Ya know, that eerily parallels a discussion that The Bride and I had recently.
In addition to the ongoing preps, she wanted to include a significant stock of both; but I think she meant only for personal/family consumption, and not barter items.
More for the list...
The value of water depends on where you are. Here in Florida its value will be minimal. Clean water will be worth a little more.
Bug repellent and bug screens will be priceless.