This section is about the exchange of money and/or items between the player and the game or between two players. "Money" can be any kind of currency, not just bills or coins. Gems, tokens, tickets, pretty much anything can be used as a game currency. Many games have multiple currency systems with some kind of restriction or difficulty preventing the two from being directly exchanged. Or sometimes they can be directly exchanged but the exchange tax/fee is high enough to discourage doing this except in special cases. Secondary currencies are used to regulate the player's time within the game; for example if the player needs 1000 casino tickets for an item they can't buy with the main currency, this encourages the player to spend enough time playing the casino minigame(s) to earn that many tickets. If the casino did not have unique rewards some players would spend little or no time playing the games there; not necessarily because the games weren't fun, but because min/max calculations told the player it wasn't the most efficient way to earn money. The players would then get bored with the game as a whole faster because they aren't experiencing the full breadth of its content. Another common example of this is PvP reward tokens which can be spent in a PvP rewards shop. Cash shop currency is a special case of a secondary currency, with the purpose of encouraging the player to contribute real money to the game in order to immediately get things that would otherwise be impossible or a lot of work for them to get within the game.
The most basic use of currency in a game is the NPC shop, which allows the player to purchase items within the game. Most literally, this is a storefront presided over by an NPC, who is ostensibly the one receiving the player's money, and adding new stock to the store as the game progresses. Functionally, NPC shops can include vending machines or automatic stores which have no NPC, and can also include NPCs which sell or exchange items or tickets but have no physical store. A simple NPC shop works like so: There is a button showing a picture of an item, and it's price. The player clicks the button to buy the item, or drags the item onto their inventory. The game checks if the player has enough money, and if they don't, an error message says so. The game checks if the player has enough room in their inventory, and again an error message will warn the player if they don't. If the player has enough money and space the item is added to the player's inventory and the money is subtracted from their wallet/money pouch/whatever the game calls the player's stock of money. Optionally you can add a step in there asking the player if they are sure they want to buy the item, or would prefer to cancel the transaction. Another option is to allow the player to buy multiple of the same item at once, by selecting or entering the quantity they want to buy. The game does multiplication to find the total price, then the rest of the transaction proceeds as usual.
An NPC shop can also allow the player to sell unwanted items back to the game. Sell-back prices are typically half or less of the shop's prices if it sells the same item. In a marketplace system sell-back prices effectively set the minimum price for which a player will sell an item to another player. Selling is just like buying in reverse - either items in the players inventory need to have sell buttons and prices, or dragging an item from the inventory onto the store should give the option to sell it. Optionally some games keep track of the last six or so things the player has sold, in case the player wants to buy them back. Some items may not have sell-back prices. They may be discardable, either by dropping it on the ground or by using a trashcan option in the inventory, but the player can't get money out of them unless they are able to trade the item to another player.
Trading is any exchange of money or items between players. As such it is not relevant to games which have no multiplayer features. (Buying from or selling to an NPC doesn't count.) Some multiplayer games do not allow this kind of exchange between players. The purpose of this may be to avoid complaints about accounts being hacked and trade-scamming, or simply because trading is off-topic to some kinds game, like those where the player has no permanent inventory or equipment; they may own items during minigames or duels, but they don't retain those items after the minigame or duel ends.
If you do want to have some kind of trading mechanism, there are two ways to make this kind of exchange relatively scam-resistant. The two methods are direct exchange (with double approval) and exchange through a shop or market, (with two-step single approval). Approval is simply when the game asks a player, "Are you sure you're okay with this?" and shows them the details of the trade they are agreeing to.
Direct exchange requires one player to click on the other's avatar or name. Some games limit direct exchange to those in the player's friends list, though I don't personally see the point of this. I suspect the idea is to discourage trade-scamming, but I don't think it actually works. Double approval is when both players add money and/or items to their side of the trade, commit to what they have put, then once both players have committed to their side, both must give their approval to finalize the trade. It's common for players to use their side of a trade to display a range of items they have available for sale, and even to show off items they have no intention of trading. For these uses it's important that they player not accidentally give away the items they are just showing. It's also common for players to add items and money to their sides in stages as they continue to chat and negotiate the trade, so it's good to allow this, and not make the trade window auto-close if the players are taking a while or the money portion be unable to handle simple addition and subtraction. Some trading interfaces include a mini 4-function calculator. The disadvantage of this type of trade is that it requires both people to be online, and some versions require the players to be standing next to each other. Also there is no way to automate offering the same trade to multiple people.
Exchange through a shop or marketplace solves the problem of players wanting to trade offline or in batches. But it is less flexible than a direct trade, because most systems don't allow mixed batches of items to be sold, nor items to be sold for anything but money. A few games have a barter system which does allow trade lots and allows the player to describe the general kinds of things they want in exchange. Barter systems are interesting but I think they are ultimately a dead end in economic evolution, which is why no one uses them in real life any more. I don't recommend adding one to a game unless it seems to fit extremely well with the flavor of the game (such as a game which chooses not to have money at all to create the feel of an ancient economy). Shops and marketplaces both use 2-step single approval: the first step is when the seller player specifies what and how many of the item they want to sell. For a flat sale they also specify the price and how long the item should be listed for sale before being delisted and returned to the player (unless the game lets sales last forever unless the player cancels them). Or for an auction they specify the minimum bid, the bid increment, and possibly an autobuy or reserve if the system has those. The seller player looks at all the information and approves it, though they may still have the option to cancel a listing if no one has bought it yet, or bid on it if the item is being auctioned. Then the buyer player looks at the item for sale and the price asked or the price they can bid, and approves that.
So what is the difference between shops and marketplaces? Personally I hate player shops, but I'll try not to be biased here. A player shop gives the player a personalized area in which they can sell multiple items for prices they specify. Some games require the player to be offline, or worse, online but idle, in order to be in shop mode. Most games don't allow prospective buyers to ask the game where they can get the cheapest currently-available one of an item they want; instead they have to look at one shop after another, hoping to find the item they are looking for at a reasonable price. So player shops are inconvenient for both buyers and sellers. The main argument in favor of player shops is that they are a realistic exchange system for a low-tech culture. An additional factor is that some players look at a player shop system and would prefer to use it like a display case instead of actually selling anything, which results in items being listed for sale at ridiculously high prices because no one is intended to buy them. That's just messy and can frustrate buyers. This can be worked around by offering players a display-case or collection gallery system which is better-suited to the purpose than a shop system. If you want some kind of player display system, I'll talk about those in the Player Property and Housing section.
Marketplaces, by contrast, have all sorts of convenience features like searching by keyword, browsing by category, sorting by price or item quantity, and the ability to choose an auction sale type in addition to a flat price sale type. In some games players must visit a physical location or NPC auctioneer to access the marketplace, while in other games it is accessible from anywhere through a menu or GUI icon. Marketplaces enable some interesting economic gameplay, like attempting to buy up all of an item to create an artificial shortage and inflate prices, then sell the hoarded items at a profit. This is illegal to do in reality, but that's no different from attacking other people with bladed weapons, stealing cars, and other things that are fun in games even though they aren't allowed in the real world. Some games try to discourage this sort of market manipulation with auction fees or other sales taxes; I'm not personally in favor of taxing marketplace use, but it's a legitimate design strategy and realistic place to put a gold sink, so it works well in some games.
Trading cash-shop currency. Many games do not allow players to trade cash shop currency, possibly because they think they will sell less cash-shop currency this way. I think it's actually the opposite; it's very difficult to convince a player to make their first purchase of cash-shop currency, but someone who is already inclined to buy cash-shop currency for their own uses can be easily convinced to buy more by the opportunity to exchange it with other players for items or in-game currency, or use it to gift other players with cash-shop currency or cash-shop items. Trading cash-shop currency is especially useful in games where many items purchased from the cash shop are soulbound to the purchaser and can't be traded at all. (The purpose of this is to ensure that players buy new ones instead of passing the same ones around from player to player within the game for years, and the same applies to games where all crafted gear is bind-on-equip).
Mail. Some games allow sending money or items through the mail (otherwise known as the private message system). This is not a trade, since nothing is received in return. This is useful if a player needs to send items from one character on his account to another, since it's (usually) impossible for the two to be online at the same time. It's also a convenient way for the game to send an item to the player, such as returning an item that failed to sell on the marketplace, or giving the player a holiday gift or subscription reward.
- Money System [This is applicable to both singleplayer and multiplayer games; only skip it if your game has no currency whatsoever.]
[indent=1]- [Describe the primary money system: What is the currency called? What denominations does is come in? How does the player earn this money? What is the player intended to spend this money on? Is there any special story explanation behind this currency?]
[indent=1]- [Describe each additional money system, if any]