Barter exchanges have their own unit of exchange, usually known as barter or trade dollars.  Trade dollars or barter dollars are valued in U.S. currency for the purposes of information returns.   Trade dollars allow barter to take place between parties when one party may not have a simultaneous need or desire for the goods or services of the other members.  Barter exchanges act as the bookkeeper for keeping track of trade dollars that participants accumulate. Earning trade or barter dollars through a barter exchange is considered taxable income, just as if your product or service was sold for cash.
Need a ride? Zimride is a ride share service which members use to set up private networks for sharing rides and saving money. Most cars fit four people, yet we usually commute by ourselves. Why not share the burden of car ownership and resource consumption? Centered around hundreds of colleges and universities, you can probably find a ride almost anywhere you need to go near campus.
However, this isn’t always possible. For instance, you may have a $150 digital music player and want a small refrigerator worth $100. In this case, if both parties are certain of what they want and understand the difference in value, there should be no barterer’s remorse. Alternatively, you can ask for the mini-fridge plus $50 to make the trade – the worst anyone can say is “no.”
Anthropologists have argued, in contrast, "that when something resembling barter does occur in stateless societies it is almost always between strangers."[6] Barter occurred between strangers, not fellow villagers, and hence cannot be used to naturalistically explain the origin of money without the state. Since most people engaged in trade knew each other, exchange was fostered through the extension of credit.[7][8] Marcel Mauss, author of 'The Gift', argued that the first economic contracts were to not act in one's economic self-interest, and that before money, exchange was fostered through the processes of reciprocity and redistribution, not barter.[9] Everyday exchange relations in such societies are characterized by generalized reciprocity, or a non-calculative familial "communism" where each takes according to their needs, and gives as they have.[10]
So as you consider items to stockpile, you should think about items that are necessary, but might not take up a lot of space.  Referring back to Mark Goodwin’s book mentioned above, one thing the main characters looked for were canning lids and rings.  They don’t take up a lot of space and in a real collapse situation, they will be really worth it!

It is said that “Necessity is the Mother of Invention.”  If we ever get to a point where a barter economy is the way goods and services are exchanged, things will come together.  But those who are prepared ahead of time, those who have thought about all of this, will be in a place where they can quickly move to make decisions that will benefit themselves and their family financially.