Those items ARE incredibly valuable. But like Coasterfan said,they were difficult to counterfeit. Paper money is useless. Prime example is the NCR is trying very hard to impliment paper money,but the Legion is using Gold and Silver coin. You actually lose money converting NCR dollars to caps.

In the Fallout universe, they chose bottle caps as a currency because they were durable and nobody had the equipment to counterfeit them or melt them down.

wow thanks coasterfan! Now i can playin harmony, knowing that my caps are there for a damn good reason

But they're useless.
And yet, every single major group all across America, no matter how much they hated or warred against each other, all accepted them as currency, no questions asked.
Also, if the gun runners can make guns, I'm sure they can cobble together some caps.

I'm guessing that Nuka Cola bottle caps were unique, as I've played about 600 combined hours of fallout and never found a machine that could make bottle caps EXCEPT for the Nuka Cola factory in Fallout 3.

DISREGARD THAT, CAPS COUNTERFEITING SHACK.

you tell me how a 100 dollar bill is physically useful, yet you can buy a lot of stuff for it

They all use it because its the accepted form. Today at work, while I was on a register, some guy wanted to use a loony to cover his balance, and I wouldn't accept it; when he gave me a dollar bill, I did. Why? Because I know I (or rather my boss) could take that dollar and go somewhere else and buy something with it, because as a society we have agreed that this piece of paper has value, whereas the loony does not (to us).

In Fallout, I do think that guns and food are king but say we wanted to trade. You have 3 days worth of food and I have an extra gun to trade. Well, mine is worth more than yours, yet I still need the food and you need the gun. Am I to just starve and you die without a gun? Should I just take the finance loss? Or we could agree on something as a proxy, (the caps) knowing that in the future I could take the extra caps and use them to cover a future trade deficit with you. Instead, I find someone else to trade with and convince him to accept the caps to cover my trade deficit, both of us knowing that you'll accept them in a fair trade. Eventually it expands to more and more people and is the accepted currency.

The same has been speculated as to the start of currency long ago.

I understand how currency works.
I'm saying that you have groups who have done nothing to each other but fight, and they all use the same damn thing as currency.
It's also unregulated.
What if some guy finds a whole shit ton of caps in an old factory?
He becomes rich as balls without ever having traded anything valuable, akin to a government just printing out fuck-tons of money to make their country richer, which, in case you didn't know, is a terrible idea.

But a market collapse should have happened.
And regardless of the problems with barter, the whole "no double coincidence of wants" or whatever, one would think that in a situation such as post apocalyptic America, it would be the dominant form of trade.
After all, a market based on barter isn't impossible, just difficult to maintain on a large scale.

Welcome to the Wasteland.

No government
No enforcement of laws outside of settlements at all
And finding bottle caps in a factory is just like finding treasure at the bottom of the ocean

Well, in some ways, caps are less susceptible to inflation than current currencies since they are, in theory, finite whereas our governments can, and do, print more money.
And as you mention, a market based on only barter is difficult to maintain in large scale. And the wastelanders have trade with the Commonwealth of Canada, not to mention all the hundreds of various tribes/bands/Republics of Dave that are wondering around.