Thinking about selling a game before any of it has been programmed may feel like getting ahead of yourself. However, the intended sales method affects the design, and plans for earning and dividing income affect recruitment of other team members. Non-profit and/or opensource games also have legal differences with regard to what resources you can use to make the game or how large of a fee you have to pay to use them.
Distribution used to be more of an issue, but these days most indie games are sold entirely online and there's no need to worry about printing CDs, packaging them, and getting them to brick-and-mortar stores. If physical sales happen for an indie game at all they usually happen through a distributor who is already selling the game online, or instead of the game itself the distributor may sell cash cards and you may sign an agreement to add your game to the stable of games that the cash card works with. If you want to sell physical spin-off items like keychains, t-shirts, stuffed animals, etc. you can either do that yourself or you can do it through an online merchant that prints these items on demand and sells similar items for many games, comics, and other fandoms; they probably get cheaper bulk shipping rates than would be available to you.
Online distribution involves either providing downloads of the game, providing a server for the player to play the game on, or both. These services are available through different kinds of distributors, such as appstores and other online game distributors, social networking sites, and server farms; it's also possible to build a server room full of machines yourself, provided you can get a location with access to professional quality internet.
There are three main ways of getting payment from players: selling a game outright (optionally after giving the player a free trial), charging a monthly subscription fee (again there is often a free trial first), or making the whole game f2p (free to play) and instead earning income through a cash shop and/or ad revenue. For a singleplayer game, the one-time sale is the standard model, except for apps. Apps are applications for smartphones or devices like ipads, where the game is sold through an appstore which can be assumed to already have a billing account set up for the device owner. (Account set-up is a traditional hurdle for all online sales because people may decide they don't want a game badly enough to fill out a form and give their credit-card number to a company they aren't familiar with. Thus, avoiding the need to create an account can result in more sales.) Apps can use a traditional one-time sales model, or they can be freemium (free or ad-supported except for premium content) and can use a cash shop to sell the premium content (including the ability to remove in-game ads) even though the game otherwise has few or no online features. Ad-removal can also be sold as a subscription, often this is an annual subscription instead of a monthly one.
For an online multiplayer game the choice is usually whether to go with a cash shop or a subscription. Cash shops are generally more popular with younger (i.e. credit-card-less) and casual players (don't spend enough hours per week playing to feel that they could make full use of a subscription), while subscriptions are more popular with hardcore players (subscriptions can limit beggars and spammers, and more importantly ensure that the game is fair to all players, not pay-to-win).
So, how is the sales method relevant to design? For starters, if your game is going to have a free trial, you need to figure out what the limiting factor(s) of that trial will be, and how to notify the player when they are encountering one of these limits so they aren't confused by why they can't do something, without spamming them silly with constant notifications.
If you intend to have a cash shop you need to design items to sell in it. Items that won't be available within the game for normal currency, items that everyone playing the game wants, but that don't make the game so obnoxious for those who don't buy them that they quit. Cash shop item sales come from three overlapping categories: Consumables, Conveniences, and Customizations.
Consumables include things like energy refills, timer accelerators, and buff scrolls, especially ones of double XP or death penalty prevention. Many of these are also conveniences. Some MMOs also require a consumable "megaphone" to be spent to speak in the world chat channel. Conveniences include anything that spares the player time, deaths and other losses, or annoyance: a permanent inventory expansion, a version of an item that usually requires 30 or more levels to equip that is instead equippable by a level 1, packs of crafting ingredients, and mounts or mount upgrades that increase traveling speed. Mounts (and limited or premium pets) are also a major category of customization, along with clothing, dye, special haircuts, and other visual effects from fireworks and petal showers to neon username frames. And again, many of those are consumables.
So the point is, if you intend to have a cash shop you have to create lacks in your game, which the player can remedy with time and/or work, or can immediately fix with a cash item, and you also have to consider how cash shop pets fit into your breeding system, and what percent of your cool hair and clothing designs should be cash vs. what percent should be quest rewards or cost game currency. It's also ideal to get players used to using the cash shop by giving them a small amount of cash currency and a few quests to buy or use cash items.
If you have a subscription system you need to have some kind of scheduler to remind people to renew before their subscription runs out. Longer subscriptions often come with bonus items, and a method to transfer these to the player's account and, if the account has more than one character, a method to let the player choose what character to receive the item on.
And on top of all that, you need to know how much money you are willing to spend on developing your game, and you need to keep track of money you spend toward developing the game so you can know later when (if) the game breaks even by earning that money back. If you are promising team members future revenues from the game, you need to have a plan for how that will work, and if that payment will end after a certain amount of money or time. If you are paying team members up front you are probably going to need a paypal account. And if you are selling your game, cash items, or any spin-off merchandise up front you are going to need a merchant account to accept credit card transactions.
You should now add a sentence about how your game will be sold and distributed to your statement of purpose. If you intend to recruit a volunteer team you may want to specify whether the game will be for profit but wait and get their input on which specific sales method to use. If you plan to make a kickstarter or similar crowd-sourcing call for funds there are extensive how-tos on that topic by people who have a lot more experience with that than me. "I, [YourName], am putting up [X$] as a budget for developing this game. [If you plan to ask other people to donate money, who and how?] The priority for what to spend this budget on will be [1st, 2nd, if any is left]. [If you are planning to offer shares or royalties, how will that work or when will you decide how that will work? Will some go to monthly webhosting bills or other game maintenance? Will some go to paying back investors, including you?] The game will earn money through [cash shop, subscription, flat sale, ad revenue, and/or spin-off merchandise sale]." If you have any ideas for specific cash-shop items or a kickstarter plan, put them in an appendix for later.
