Originally, these rules were written for the upcoming seacrawl, Our Mother, the Sea, but they may be helpful for any game where trading is a focus of play.

A few notes: 

• Item Classes refers to groups of similar products. In Our Mother, the Sea, there are six: Provisions, Fuel, Sundries, Weaponry, Trade Goods, and Ship Parts.
• Market Capacity amounts likely need to be adjusted depending on the ruleset.
• A Settlement Turn is currently 2 weeks in Our Mother, the Sea, but this could easily be increased to 1 month or more if settlement economies are meant to be stable.

Economy

Each settlement has an Economy Tier, which ranges from I-VI. The Economy Tier determines the Market Capacity, referenced on the table below: 

Economy Tier	I	II	III	IV	V	VI
Market Capacity	100 sp	500 sp	1000 sp	2000 sp	3000 sp	4000 sp

Trading

Rarity

Each item has a Rarity Score, which ranges from 1-5. To determine if the item is available for sale, roll a number of six-sided dice equal to the Economy Tier. For each dice that exceeds the item’s Rarity Score, there is a unit of the item available.

Items without a Rarity Score are always available in decent quantities.

Re-roll each Settlement Turn.

Buying

When the characters buy products from a settlement, keep track of the amount spent for each Item Class.

If the amount exceeds the settlement’s Market Capacity, then the Item Class becomes in Demand.

If the Item Class was in Demand already, no products of that Item Class may be bought. Alternatively, if the Item Class was in Supply, then it becomes no longer so.

Selling

When the characters sell products to a settlement, keep track of the amount sold for each Item Class.

If the amount sold exceeds the settlement’s Market Capacity, then the Item Class becomes in Supply.

If the Item Class was in Supply already, no products of that Item Class may be sold. Alternatively, if the Item Class was in Demand, then it becomes no longer so.

Supply & Demand

Supply

Item Classes in Supply are valued at half the price they normally would be. Roll an additional dice when determining if products in that Item Class are available for sale.

Demand

Item Classes in Demand are valued at twice the price they normally would be. Roll one less dice when determining if products in that Item Class are available for sale.

Settlement Condition

At the beginning of each Settlement Turn, 2-in-6 chance the settlement experiences an economic shift. Determine the affected Item Class randomly, then roll 1d6. On a 1-3, the Item Class becomes in Demand. On a 4-6, the Item Class becomes in Supply. If the item was already in Demand or Supply, then it instead becomes Neutral.

At the end of each Settlement Turn, compare the number of Item Classes in Supply and Demand:

• If the number of Item Classes in Supply exceeds those in Demand, increase the Economy Tier by 1.
• If the number of Item Classes in Demand exceeds those in Supply, reduce the Economy Tier by 1.

Example Settlement

Commune of Mary-sur-Mer

Economy IV (+Weaponry, -Trade Goods)

Strong is the wall round the town of Mary-sur-mer. Stronger are the 600 who live there. Both have weathered the worst of man and sea.

Mary’s Gate blocks the fjord that runs from town to the sea, manned at all times by well-armed 2d6 Communist Soldiers. The gate swings open easily for any red-painted ships that approach. Others are treated with harsh suspicion.

The sawmill whines from dawn to dusk most of the week, but its music has grown sparser of late. Logs arrive without their drivers from Camp Normandin, and among the loggers there is talk of a blood-drinking bear. The locals scoff at such superstitious claims. The River Vert is ever fickle, they say.