It's probably unnecessary to elaborate this much but I want to be clear.
It seems to me that people here are discussing 2 issues, prices and method of exchange.
By method of exchange I mean barter or money. Barter is an exchange of goods (eg cavebulbs for armor), money is medium of exchange, one party provides or accepts money (currency) in exchange for goods (buying or selling). In barter I directly trade what I want (say copper plate armor) for what I can provide (cavebulbs, cheese, etc), if I'm using money I would sell my bulbs/cheese for money then use that money to buy the copper armor, the act of buying and selling is separated.
Prices are the quantity offered in goods or money, either I trade the armor for a quantity of one or several goods or I buy it with coins. The reason use of money developed in the real world is because it avoided the
problem of 2 people wanting 2 unique types of good and having what each other wanted as a prerequisite of exchange.
Barter is largely the rule of the game because the benefits of money aren't yet worth the inconvenience of obtaining it, debts are rarely incurred, exchanges are few and can be pre-arranged via the forums or irc, and we have fast-travel. Large villages want almost any type of good produced, negating the coincidence problem. People trade for what they use, rather then saving money (also a store of value) which could, as Thurrok said, be largely worthless. Good like cavebulbs are traded often so the prices are stable, whereas metals are used as money less often by smaller number of players and prices are less definite.