Economist here. You are right that during periods of hyperinflation, people turn to other forms of exchange including bartering and alternative currencies. Even when there is no hyperinflation, there may be a shortage of currency that leads people to seek other forms of exchange. An example of currency shortage is the Russian Ruble in the last years of the USSR and especially in former Soviet states shortly after the fall of the Soviet Union. A third cause of other forms of exchange is reluctance to use a currency; either to avoid taxes or price controls or because the two parties involved do not share a common currency - which can occur in business exchange, too. In more recent times, dollarization is another alternative to bartering. For example, during hyperinflation in Argentina in 1989-1990, while the Argentine Austral (they dropped the Peso from 1985-1991) lost value, there was enough access to US dollars that people used dollars when they could. Dollarization is, to me, the closest thing to a hypothetical turn to precious metal trading. In most cases, the unit of barter has usefulness to other consumers, while foreign dollars (and precious metals) have value to central banks but not to other consumers (so if the central bank collapses or is far away, barter becomes more likely than dollarization or precious metal usage).

The common form of barter is, as you say, exchange of a good that has some value in the community. In most cases, the major good of exchange is something that has some intrinsic value for the community. This can lead to a de facto currency, such as potatoes in Bulgaria in 1997 (Cellarius 2000) or cigarettes in WWII POW camps (Radford 1945) and US prisons (yes this occurred but use declined when restrictive smoking policies became more common in prisons starting in the 1980s [Lankenau 2001], although a friend of mine who was in jail a few years ago said they used Little Debbies). In the Russian case, coffins, bras, and even rubber dildos were reported to be used as currency (The Economist 1997 - FWIW I'm skeptical of this and didn't find any other sources supporting sex toy currency). Cash can also be cut out of contracts, so an urbanite would grant a farmer the use of land in exchange for a share in agricultural output (in the form of crops or livestock) at the end of the season (Cellarius 2000). Economists say that money needs to have a number of characteristics including being a medium of exchange and a store of value. Thus, a perishable good is not likely because its value decays and is not stored, but non perishable foods (like potatoes) can be.

Cellarius (2000) give s good description of a the use of potatoes as currency in Bulgaria in the late 1990s and gives a description of a situation that fits well into economic theories of barter and alternative currencies. In practice, barter usually requires a relationship between the two people exchanging goods, something not necessary for currency exchange. This is because barter requires a "double coincidence of wants", something which is often only partially true. Since one party of the barter may not be fully satisfied by the exchange, the existence of a relationship between the parties ensures that they feel comfortable that they can get similar terms for another exchange later when the terms may seem more advantageous. For related reasons, gift giving is another form of exchange which arises in such economies (with gifts ultimately reciprocated in some form). As there are heavy reputational requirements, these economies often privilege exchange among people who live near to each other.

As such, exchange within small Bulgarian villages was more likely to be barter or using potatoes as an alternative currency, while exchange between villages was be more likely to use cash. Finished and high value agricultural goods like dairy and livestock also were used in inter-city and inter-village exchange. Also, someone making an exchange between villages was more likely to be wealthy and thus have other ways to access cash not available to poorer individuals.(Cellarius 2000)

I will skip the question of prostitution except to say that it does not strike me as an efficient form of alternative currency, and more likely stems from increase need or poverty during a crisis. That is, people may sell sex for goods, but it isn't likely that someone selling goods prefers sex as payment over cash or other goods (for more, see Julia Roos (2010); Roos is the leading scholar on prostitution during Weimar Germany).

For a bit of commentary as a conclusion, the evidence suggests that a precious metals are less likely to be used for exchange during a crisis than a durable, useful good such as a food that rots slowly. But to me, the prepper community is supposing a new economy arise between preppers, and if precious metals are valued as currency in that community, fair enough, makes as much sense to me as cowrie shells.

Sources:

Cellarius, Barbara A. "" You can buy almost anything with potatoes": An examination of barter during economic crisis in Bulgaria." Ethnology (2000): 73-92.

Eeonomist, The. 1997. The Cashless Society. March 15:77-78 - https://www.economist.com/finance-and-economics/1997/03/13/the-cashless-society

Lankenau, Stephen E. "Smoke'em if you got'em: Cigarette black markets in US prisons and jails." The Prison Journal 81, no. 2 (2001): 142-161.

Radford, Robert A. "The economic organisation of a POW camp." Economica 12, no. 48 (1945): 189-201.

Roos, Julia. Weimar through the lens of gender: prostitution reform, woman's emancipation, and German democracy, 1919-33. University of Michigan Press, 2010.