This mod implements marketplaces where players can post buy and sell offers for various items, allowing for organic market forces to determine the relative values of the resources in a world.

The basic market interface is the same across all markets and market types, but this mod allows for a variety of different ways that markets can be configured to support different playstyles. Markets can have restrictions on what they will allow to be bought and sold, different types of "currency", and can share a common inventory across multiple locations or can be localized to just one spot at the discretion of the server owner.

Currency

Each market has one or more "currency" items defined that are treated differently from the other items that can be bought and sold there. Currency items are translated into a player's currency balance rather than being bought and sold directly.

For example, some of the markets offered by the "commoditymarket_fantasy" mod have this currency definition:

{
    ["default:gold_ingot"] = 1000,
    ["commoditymarket_fantasy:gold_coin"] = 1
}

When a gold ingot is added to the player's market account it turns into 1000 units of currency. When a gold coin is added it turns into 1 unit of currency. You can't buy and sell gold directly in this market, it is instead the "standard" by which the value of other items is measured.

There's no reason that all markets in a given world have to use the same currency. Having variety in currency types adds flavour to the world and also introduces opportunities for enterprising traders to make a profit by moneychanging between different marketplaces.

Account Inventory

In addition to tracking a player's currency balance, each player's account has an inventory that serves as a holding area for items that are destined to be sold or that have been bought by the player but not yet retrieved. This inventory is a bit different from the standard Minetest inventory in that it doesn't hold individual item "stacks", allowing for larger quantities of items to be accumulated than would otherwise be practical. If a player needs to buy 20,000 stone bricks for a major construction project then their account's inventory will hold that.

To prevent abuse of the market inventory as a free storage space, or just to add some unique flavor to a particular market, a limit on the inventory's size can be added. This limit only affects transfers from a player's personal inventory into the market inventory; the limit can be exceeded by incoming items being sold to the player.

Note that tools cannot be added to the market inventory if they have any wear on them, nor can the market handle items with attached metadata (such as books that have had text added to them).

Placing a "Buy" Order

A buy order is an offer to give a certain amount of currency in exchange for a particular type of item. To place a buy order go to the "Market Orders" tab of the market's interface and select the item from the list of items on the market. If the item isn't listed it may be that the market is simply "unaware" of the item's existence; try placing an example of the item into your personal inventory and if the item is permitted on the market a new entry will be added to Market Orders.

Enter the quantity and price you desire and then click the "buy" button to place a buy order.

If there are already "sell" orders for the item when you place a buy order, some or all of your buy order might be immediately fulfilled provided you are offering a sufficient price. Your purchases will be made at the price that the sell orders have been set to - if you were willing to pay 15 units of currency per item but someone was already offering to sell for 2 units of currency per item, you only pay 2 units for each of that offer's items.

If there aren't enough compatible sell orders to fulfill your buy order, the remainder will be placed into the market and made available for future sellers to see and fulfill if they agree to your price. Your buy order will immediately deduct the currency required for it from your account's balance, but if you cancel your order you will get that currency back - it's not gone until the order is actually fulfilled.

Double-click on your order in the orders list to cancel it.

Placing a "Sell" Order

Sell orders are an offer of a certain amount of an item and a price you're willing to accept in exchange for them. They're placed in a similar manner to buy orders, except by clicking the "sell" button instead of the "buy" button.

If there are already buyers with buy orders that meet or exceed your price, some or all of your sell order may be immediately fulfilled. You'll be paid the price that the buyers are offering rather than the amount you're demanding.

If any of your sell offer is left unfulfilled, the sell order will be added to the market for future buyers to see. The items for this offer will be immediately taken from your market inventory but if you cancel your order you will get those items back.

Double-click on your order in the orders list to cancel it.

Commands

This mod has several commands that a server administrator can use:

• market.removeitem marketname item -- cancels all existing buy and sell orders for an item and removes its entry from the market tab. This is useful if you've changed what items are permitted in a particular market and need to clear out items that are no longer allowed.
• market.show marketname -- opens the market's formspec
• market.list -- lists the marketnames of all registered markets
• market.purge_unknowns -- executes "removeitem" for all markets on all items that don't have a definition. Useful for clearing out items that are no longer defined due to a mod being updated or removed.
• market.addeverything marketname - Adds all registered items to a market's listings. NOTE: this is intended as a debugging tool, not for use in a live server, as it doesn't filter out items that a player cannot actually harvest in-world.

Registering a market

The mod "commoditymarket_fantasy" contains a number of pre-defined markets that provide examples of what's possible with this mod. They include:

• King's Market - a basic sort of "commoner's marketplace", only open during the day
• Night Market - the shadier side of commerce, only open during the night
• Trader's Caravan - a type of market that players can build and place themselves, with a small inventory capacity.
• Goblin Exchange - a strange marketplace that uses coal as a currency
• Undermarket - where dark powers make their trades, using Mese as a currency

All of these except for the Trader's Caravan are intended to be placed in specific locations by server administrators or mapgen, they don't have crafting recipes. Modifying these markets or creating your own from scratch should hopefully be a fairly straightforward task.

Market definition API

local market_def = {
    description = "Night Market", -- A short name for this market, appears as the text of the "info" tab of the market's UI
    long_description = "When the sun sets and the stalls of the King's Market close, other vendors are just waking up to share their wares. The Night Market is not as voluminous as the King's Market but accepts a wider range of wares. It accepts the same gold coinage of the realm, one thousand coins to the gold ingot.", -- A longer description with flavor text and other information to present to the user, shown in the info tab. Optional.
    currency = {
        ["default:gold_ingot"] = 1000,
        ["commoditymarket:gold_coins"] = 1
    }, -- List all items that get translated into "currency" here, along with their conversion rates. Take care to ensure there's no way for a player to multiply their money when crafting currency items into each other (eg, if there was some way to get more than 1000 coin items out of a gold ingot, in this case)
    currency_symbol = "☼", -- Used in various places in the UI. If not defined, defaults to "¤" (the generic currency symbol)
    inventory_limit = 10000, -- Optional, when set this prevents the player from adding items to their market inventory when it's over this limit
    sell_limit = 10000, -- Optional, when set this prevents sell orders from being added if the player already has this many items for sale
    initial_items = {"default:cobble", "default:wood"}, -- Optional, a list of items that the market will be initialized with on startup. Players can add other items during play.
    allow_item = function(item) return true end, -- Optional, this function is used to determine whether the market permits a player to add a particular item to its inventory.
    anonymous = true, -- If set to true then the player won't be able to see the names associated with other player's orders, only their own.
}

commoditymarket.register_market("market_name", market_def)

Once a market is defined, use commoditymarket.show_market(market_name, player_name) to show the market interface to a player.