The Great Zimbabwe and the Design of Active Pricing in Passive Trade Systems

Passive trade systems in tabletop games solve a big problem: trade is fun, but negotiations can slow gameplay. In a passive trade system, some players offer resources for sale, and others can buy them unilaterally. Some systems require players to offer resources at a fixed price, like open production buildings in Imperial Settlers. Others let players choose their own prices, like the Riverfolk Company in Root. Letting players choose prices sounds interesting, but it can lead to degenerate gameplay where players always set the maximum or minimum price. To make active pricing work, you need restrictions that create incentives for both high and low prices, stabilize prices to be somewhat predictable, and allow competition between players charging different amounts.

The Great Zimbabwe has perhaps the best passive trade mechanics I have seen in any game. Trade works as follows:

• You build craftsmen, whom you use to construct monuments to earn victory points.
• Once you build a craftsman, anybody can pay cattle to use it, including you.
• Using a craftsman consumes a resource from the board.
• All cattle paid to a craftsman go to their owner at the end of the round.
• You can set the price of your craftsman between one and three cattle.
• You can only change your prices when building a new craftsman.
• You can only increase prices; you can never decrease them.

To start with, because everybody must pay the same cost to use a craftsman, you cannot just charge the maximum price by default, because you will have to pay that price as well. Yes, you get the cattle back, but only at the end of the round, which means your actions during the round may be more limited. At the same time, charging the minimum price means you will get fewer cattle from other players' purchases. This rule alone prevents degenerate pricing strategies.

Since craftsmen require resources from the board, you can compete with other players even if you offer different prices for the same type of craftsman. One placement rule in The Great Zimbabwe is that each craftsman must have access to at least one unique resource not used by another craftsman of the same type. You might charge three cattle for ivory while another player charges two, but players may still buy from you if your competitor has run out of resources and you have not.

The strict limits on price changes make the prices you choose much more meaningful than if you could change them every turn, because after you set them, you will be stuck with them for a while. Building new craftsmen is very costly because not only does it consume cattle, it also takes an entire turn you could otherwise be using to build monuments and score points. The difficulty of changing prices also creates market stability, helping other players plan their turns without worrying about constant pricing changes.

Prices can only increase, never decrease, creating a natural inflation arc: prices start low and inflate over time. This adds weight and permanence to pricing decisions: you are limited in when you can change prices and cannot reverse them. Low prices create scarcity that discourages turtling. If another player offers a one-cattle sale, you may want to buy now because they might permanently raise their price later.

Taken together, the many restrictions on craftsmen in The Great Zimbabwe create a compelling passive trade system that does a pretty good job of simulating an economy. If you want pricing decisions for your passive trade system to be meaningful, you need to include enough restrictions so players can't always charge the highest or lowest price every turn. When designing your pricing system, ask yourself the following questions:

• Is it possible to charge a higher price than an opponent and still get customers?
• How often do players change their prices?
• How often do players charge the maximum allowed price?