One of the first things to keep in mind is that an economic system is not a money system. A money system is part of the economic system, but economy is so much more. An economy is about how the resources of a world are exploited and how the wealth created by that exploitation is spread around. Closely related to the economic system is the political system because that describes how the people that deal with all this wealth and activity are organized, and the two are always closely tied together.
As human history has shown there are many, many ways to organize both economic systems and governments. What is important to your believability is internal consistency. Lorex Camera Setup
For example: suppose you have taken time to describe a Stone Age family living in a grass hut by the beach as part of a Stone Age village.
OK… then we read…
Momma hears a ring tone, whips out a cell phone, listens, and says, “Papa, it’s the Joneses. They have half a dozen lobsters to trade.”
“Tell them we’ll give them two pearls, and not a coconut more.” grunts Dad while he is mending nets with the other men who are at the hut.
What happened here? Is it believable?
What we have here is a mix of technologies, and that mix implies a difference in economies. The cell phone is not coming from the Stone Age, barter-based economy you have been setting up.
Is it believable?
It can be if your story includes elements of culture differences as part of its theme, or perhaps a touch of humor. This could be part of a story about the lives of villagers next to a new alien city that has sprung up recently, or something ala The Flintstones. But if this is a dramatic fantasy story about vikings, sirens and maidens, this scene is going to be tough sledding. Apppon