Barter sets in quickly. The unvarying long-term demand is for high calorie food, especially canned meat. Rice and flavorings are popular as well. MREs are good as gold. Not far behind is comfort stuff, coffee in particular, and tobacco, cigarettes mainly, traded by ones and twos. Also near the top of the list are candles and batteries in common sizes. Oddly, medical supplies, guns and ammunition aren't often mentioned.
That is because guns and ammo may not be for sale or trade at any price. However in Iran after the fall of the Shaw- .22 ammo was commonly used in trade, so I am not sure how accurate that statement is.
But then these stories are always from countries with no tradition of individualism, certainly without traditions of personal firearm ownership and in societies far less pampered and removed from the harsh realities of the world, and usually without multiple levels of government ... that may make our results vary, or it may not. When the fed fails there will still be State, County and town governments to fall back on, and depending who is running those your particular area may or may not be co-ordinated (and depopulated) enough to defend itself without going through the "which gang gets control" phase.
Large swaths of the population will lay down and die, be wiped out, or be enslaved to the gangs that will form in the areas where the local governments fail. But the general weakness and stupidity of the average citizen will render the vast majority harmless and starving. I agree the better option is to "get out" but I don't think Americans will be readily accepted as refugees in most places in the world, and their governments are as likely if not more likely to become more despotic as Russia and China claim hegemony over the planet.
My key really is to try to get into a position where I am mostly trading with neighbors known to me with whom I band for mutual protection. I need to go out and just stock up on a huge crate of cigarettes. Our individualism and heavy armament will probably change how this goes a bit. Granted we aren't going to be attacking armored divisions or dealing with air supported units directly - but the terrorists overseas don't either. That is what IEDs are for. And these troops won't be in well protected "green zones" - their families and relatives will be vulnerable to retaliation.
War gaming this in many ways pointless. We really don't know how it will go down, other than it will be ugly. Be it just a controlled slide into a corrupt 3rd world banana republic, or full on Mad Max social anarchy. I suspect it will be closer to the former, with the mad max stuff happening in mostly urban areas with "yutes"-- But a mass die off will occur and we will loose the most useless people to ever walk this earth, , and then - well really there is a lot of open country, a lot of competent people hell bent on individual freedom and as things settle down , probably a lot less reason to fight and more reasons to co-operate. The ultimate objective of removing America form the world stage will be achieved by Obama's masters, but I really doubt they bother to try and control this nation after they have done so. Too much work for way too little gain. Of course they might just scortch earth the continent and kill everyone.. but no amount of prepping will prepare you for that, and being else where means you will just be enslaved if you survive so, yeah.