I harken back to the days of Mountain men and the Hudson's bay company and Rendevous.

Barter seems to be the fallback market, with practical goods being the more common exchange mecium. Precious metal minted into coin of the realm still seems to be a viable currency if one must travel and still do business. Ammunition might be suitable for a time, but it's purpose predisposes it to eventual consumption. Other items being more awkward to ferry about, it seems inevitable that some currency, not easily manipulated, fairly securable, durable, and easily recognizable, would have to manifest sooner or later.

I envision that once the initial shock of crisis is over, people will gather to exchange what they have in abundance with what they need or are want for. If folks cannot find the items they desire at one location, they are bound to travel, and will not be willing to encumber much. For this reason, I suspect a sachet of gems and coins, as in old times, will take the place of sacks of grain and sides of beef. Money is just too convenient an exchange medium, but people will not accept notes after this.

Gold and silver cannot be counterfeited. An ounce of either can be proofed readily enough, and once a minted coin receives a proof mark from a reputable local agent, it should exchange fairly routinely. In fact, for this reason junk silver is a good investment pre-collapse. It is already proofed satisfactorily that anyone can determine it's relative value. It is in portions that allow for easy whole division for market. I can easily imagine that in short order you will find traveling vendors peddling goods by coach such that he can deliver to a neighborhood things a man afoot would have trouble retrieving from any distance. While he may find barter items suitable to stock (produce from a home garden, tools, trinkets, and so on), he will also be quite interested in exchanging for common and highly fungible resources.

It may not happen right away, but depending on how bad things get and how long they stay that way, the practicality of secure currency seems inevitable. There may be a time when only material goods will trade fairly. But sooner or later, some standard must be recognized and adopted.