Trading

The Warframe trading system was added in Update 11. It allows players to trade Mods, Platinum, and Keys.

Requirements

Open your Keys menu on the main screen.
Select your "Clan Key" and use the key.
Press "Play now".
Once inside your dojo, use the invite command "/inv name" to invite your trading partner. Alternatively you can right-click their name on the chat window and select "invite".
Navigate to your Dojo's Trading Post and when the option appears activate it with your "use" key.
Select your trading partner from the list, and click "Trade".
After your partner accepts, you will see 3 trade slots for each player. Click one of the trade slots to select the item you wish to trade. It will give you the option of choosing Mods, Platinum or Keys.
Repeat for the other two trade slots if necessary.
When you are finished with your side of the offer, click the "Offer" button.
Your trading partner will now be able to review what you have offered, take the time to make sure you are getting what you asked for, it is easier to prevent a scam or honest mistake than it is to fix it.
Your trading partner will now have the ability to select their 3 trade items. The process is the same as above.
When your trading partner has finished and clicks "Offer" as well, you will be able to review what you have offered, and what your trading partner has offered. Take the time to make sure you are getting what you asked for, it is easier to prevent a scam or honest mistake than it is to fix it.
When you are satisfied with what you are both offering, click "Accept Trade".
The trade is now complete!

All trades will be charged a trade tax. Tax increases based on rarity of the mod, or tier of the key.

The tax on mods is 2000 credits for common, 4000 for uncommon, and 8000 for rare.
The tax on keys is 2000 credits for Tier 1, 4000 for Tier 2, and 8000 for Tier 3.
The tax on platinum is 500 credits for 1 platinum. (e.g. 10 platinum would be 5000 credits)

Number of trades per day:

The number of trades you are allotted each day is directly tied to your Mastery Rank. A rank 5 will get 5 trades per day. Founders get 2 bonus trades per day.
Your available trades reset each day at the same time as revives and the daily login reward (currently 7pm EST).

How to find trade partners:

You can find trade partners by browsing the Warframe Forums Trading Post on the official forums.
You can also use the in game chat tab "Trading".

Common terminology:

You can not trade the 50 Platinum you recive at the beginning.