As a nation, we are faced with a host of different problems from many directions, both domestically and internationally.  The statistical improbability of a disaster occurring (such as war or economic collapse) decreases with the passage of time and the addition of other factors that lead into such.  For a couple of good "primers" on collapse and warfare (overall effects on societies and civilizations), I recommend two by Jared Diamond: "Guns, Germs, and Steel," and "Collapse."

When any kind of society or civilization becomes unraveled, usually the nation's cash loses its value within days at the most.  We're going to cover a few general categories of items to keep for barter (meaning "regular" or frequent trade), citing individual examples within each category.  In The Prepper's Blueprint, the idea of bartering was covered extensively and it isn't always acquiring tradable goods, but also tradable services.