I believe that they would find something else of value to trade. The value of whatever would be traded would be determined by what happened in the economy to cause the traditional items to disappear. For example our current monetary system is built on "paper" money. There is no physical difference in what makes a $100 dollar bill or a $1 dollar bill other than what's printed on the surface. It's our belief that there is a greater value in the $100 bill or that we will be able to exchange it for goods or services of equal value. If our government or our "way of life" were to collapse - then the value of the cash we use now will be worthless.