Trading

Trading was introduced in version 0.44 and updated in 0.442.

Trading is a type of mission that can only be performed on a caravan (whenever a caravan is present) or at the other faction's fortress (when the player has a City Hall). When a trader's caravan appears (for a limited time), the player can select the grid that caravan is on, then select the mission. Currently the game supports trading weapons, nonweapons, craftable items, and resources. Note that Gustav only exists as a caravan and does not have a fortress of his own, and that different factions will sell different items. (Example: Riffs will sell more ammo, and the Pig farmers will sell more food.)

Requirements

 * Time: 1 day
 * Danger: varied, depends on(I believe there is zero danger doing trade missions)
 * Amount of zombies.
 * Distance to the nearest claimed block.
 * Benefits from:
 * Leadership skill (better at haggling)
 * Scouted blocks (reduces danger?)
 * Amount of survivors (reduces danger)
 * Total Defense skill (reduces danger)
 * At least one survivor has a vehicle equipped (ignores danger from distance)
 * Negotiator perk(may gain extra resources)

Note. AFAIK difficulty doesn't come into play here just as it normally doesn't for scouting. Even on impossible difficulty across the whole map in an non-scouted tile I've never seen any real danger on a trading mission.

Effects

 * Exchange unwanted resources/items for needed ones.
 * Improve Leadership skills.
 * Change Faction's power and Respect for the player.

"Pre-trade" screen
Before the actual trade screen appears, the player are presented with three dialogue options:
 * Trade
 * Ask what the trader needs (requires Leadership 5). The game then performs a hidden check if the faction is short on any resources, and if the trader decides to tell the player. The "wanted" resources can be sold for doubled value.
 * The game remembers the results of the above check for a short while. Theoretically you can leave and come back with more of what they want.
 * Leave

Player's side

 * The uppermost section shows the current "mark-up" rate (in percentage), and chance to haggle for a better one (in text). When the mark-up rate is displayed in red, it means the trader's goods are sold for higher than the base price. When it's in green, the trader's goods are sold for lower than base price. If the player haggles, the mark-up will increase or decrease by 5%, 10%, or 15% (possibly depending on the trading character's Leadership skill). For high-Leadership, 25% decreases have also been observed. The player can Haggle up to 3 times, but requires Leadership 3.
 * AFAIK 15% changes are not possible at all for higher leadership. +5% mark-up and +/- 10% mark-up do happen.
 * The middle section shows the player's resources and items. Left click selects the items to offer. For resources, a slider appears to let the player determine the amount of resource to offer.
 * The bottom line shows the value for the player's offered goods.

Trader's side

 * The uppermost section shows factions Strength and Respect for the player. Respect possibly affects the markup rate as well as chance to successfully haggle.
 * The above is incorrect. Respect does affect mark-up, but affects the starting mark-up only. For impossible with shop clerk . Probably it's 125 - respect without shop clerk, and [less than 125] - respect on lower difficulties.
 * The middle section shows the trader's resources and items. Left click selects the items to buy. For resources, a slider appears to let the player determine the amount of resource to buy.
 * The bottom line shows the total value for the trader's goods.

Trade Button

 * Usually, this button is only click-able when the player's offered goods has equal or higher value than the trader's. Once the player click the button, the deal is finalized. A event screen will then pop up, summing up the transaction and changes in the faction's strength and attitude. If the value of the player's is higher than the trader's, the faction's respect will increase.
 * If one or more characters sent on the Trading mission has the Negotiation perk, then the trader can sometimes be convinced to throw in something extra at this stage. The player can choose an item from the presented list or choose that the player doesn't need anything.  Choosing that the player doesn't need anything results in a larger-than-usual increase in Respect with the faction in question.
 * The negotiation perk triggers every time there's a negotiator on the mission, but only if certain requirements are met. (Used to trigger all the time regardless of requirements in earlier versions, if memory serves.) The player can choose food, fuel, materials and nothing. For the first three the resource in question needs to be present in traders stock both initially and after the trade. So you can't purchase all of it and ask for more or can't ask for some back if they had none originally even if you sold them some. The fourth will be available as long as the events triggers. But the event will not trigger if none of the first 3 are available.

Player's Guide
Also see impossible guide.

Raising respect
Trading is the best way to raise a faction's respect, as meetings with factions can cost a lot of time and valuable resources and can even turnout badly. Giving them extremely good deals or choosing "Nah, they can keep it." if the trader has the negotiator perk can greatly increase respect.

Trade with food
Try to trade with food. In middle stage of the game, if you have a good scavenging team you often end up with a lot of materials and equipment. But don't be too eager at selling them, because scavengable resources on the map are limited and in late game it is harder to come by these resources. Same goes for ammunition, medicine and fuel. Try to research farming techs, convert useless buildings into farms and deploy more farmers(After you have scavenged the whole map). What I am saying here is not that you cannot buy food when survivors are starving. But in the long term, exchanging other goods for food won't solve the problem and will drain other resources along with it.

Also, try to keep at least one equipment of each type, some factions might want certain equipment.

Factions-Specific Guides

 * The Pig Farmers only sell food. If you are looking for something else, don't go to them.
 * The Pharmacists sell drugs called bath salts. If there are any bath salts in storage, soldiers will immediately consume them and have a high chance to become addict. (If you ban bath salt with policy, they won't be consumed, but you can still buy and sell them)(negative effects can happen if addicts can't get bath salt.) In Abbostsford, there's a drug dealer plotline in which you can buy bath salt at a price of 0.5 each and sell at 2.(prices at other cities unconfirmed.)