Well of course for two people to trade when only one of them has what the other wants... that's where the need for money/currency arises. Something they can both agree on the value of, and can be used by the seller to later buy what he wants that the original buyer didn't have. I don't know if the money agreed upon will be silver, but that makes more sense than defunct paper money (although some will probably accept that out of habit or sentimentality). Some early forms of currency will probably be things with universal barter value, like bullets, matches, cigarettes, liquor, toilet paper. Then later things with intrinsic non-utilitarian value like precious metals or gems, eventually evolving into stuff with only symbolic value, like our Federal Reserve Notes which require faith in the issuer.