When you guys start out, you will have like 50 plat. Note that these starting plat is not tradable, so do w/e you want with your starting plat. When you begin the game as a newbie, there is a few things you will snatch when progressing through the story: Mods, Prime parts, and ship parts. in example:

Mods: Berserker, Condition Overload, Etc.
Prime Parts: Latron Prime Blueprint, Lex Prime Blueprint, Akstilleto Prime Receiver, Etc.
Ship Parts: Mantis Fuselage, Etc.

Be sure you do not sell anything for credits as this will help you hoard for platinum in the next section.

when you first start out to trade you got 2 options. The Trading Chat and the Warframe Market. For those that do not know what the Warframe Market is, the next section has a video with a tutorial or guide on how to use the Warframe Market. I mainly use Warframe Market, so the remainder of this guide will be mostly when you are using the Warframe Market. The trade chat is okay, but how often are you going to be checking the chat? Pretty sure you do not want to spend time sitting there staring at it when you can be farming for plat gain items right? Also note the Warframe Market is frequently being updated so features on the video will change.

For the Warframe Market Website: https://warframe.market/

I have a video here that talks about what exactly you can sell. For those that don't like to read.
When selling items, be sure to note the supply of it. In example how often does DE give it away for free or what updates DE did to make that specific item useless to the point where it is not worth buying at all. An example of this is a new weapon that makes its mark on Warframe, it is so freaking good that rivens are worth putting on. Guess what, the rivens themselves are worth 3000 plat. In another hand, an old weapon of some sort like a Bronco, with very good stats on the riven itself, but terrible weapon in general regardless of stats. The price of that riven would mostly go for 10-50 plat. Catchmoon weapon itself is also another example, back in the day the kitgun was so good everyone used it. With the nerf of the weapon now, it seems Catchmoon riven prices dropped.

Onto what to sell now, so when it comes to mods. I have a high success rate or high customer influx with the following:

Acolyte mods of all types: So every mod acolytes drops, apparently they sell for 5-100p based on what they are. The amount of demand for them is high because Acolytes tend to come once a year it seems based on observation. So be sure to snag em when you can, and do not sell the spares. 5 plat is better than 20 endo or 500 credits
Berserker: It is pretty obvious why, the chance of obtaining this is pretty slim, but I do get spares here and there from time to time. Ranges around 10-15. Price shifts too much but demand is high.
Condition Overload: hard to get price is same as Berserker.
Rank 8 Mods: This sound weird but any mods that has a high rank or uses way too much endo tends to sell well.They got to be maxed out though. Redirection/Primed Mods are great examples here. They normally around 50+ plat. Price changes based on supply.
Sabot Rounds: Hard to get and is a good mod for Archguns. Only obtainable via Fortuna's Heist mission. Price based on Supply, but mainly around 50p +.
Rivens: Pretty obvious why. Price changes based on supply. Pistol Rivens are the cheapest ones though.
Set mods: like Augur secrets or Vigilante fervor, these have high demand even though they are kind of easy to get on some set mods. So may need to check on the charts to see when it is a good time to sell.
Corrupted: mods from either the Orokin derelict or Baro basically tend to have high demand. Prices change so they can be really cheap or expensive.

keep in mind these are just examples, as I tend to sell lots of other things. Based on these examples you can see that any mods that takes time to get tends to sell fast regardless of the price.

Onto Prime Parts:
Vaulted: So it is pretty obvious all vaulted items are worth selling since the supply is low.
Not Vaulted: These primes will be vaulted in the future, up to you if you want to instantly sell or wait until it is vaulted. For me personally, I waited.
Can't be vaulted: primes like Lex or Braton won't be vaulted at all. Reasons is unknown but based on statistics, it never went into the vault and in my theory I think it is so because they are like the first primes or something I think or starting primes. So I assume they will never be vaulted. Just change these to ducats instead of selling them for plat.
AkLex: good amount of plat if you got the links. Ranges 50-60 plat.

Onto misc parts:
Shedu: I tend to sell Shedu currently because it is hard to get and takes awhile to get. This may change in the future, but for now at the time of writing this guide. It has a high demand of it.
Ship parts: Like Mantis Engines of Fuselage. The credits gained from selling is tempting, but I tend to sell it for 5-10 plat because they are hard to get or takes awhile to get. The demand however is low, since ships are mainly for support charges and fashion.
Weapon Parts: Vandal, Wraith, etc. These parts do get high demand because invasions clock in differently ones. So price is pretty low however due to how easy it is to get. Just similar to Shedu, have to wait for it to pop.
Ayantan Scupltures: Endo is hard to get. So its pretty obvious its cheaper to buy endo from Ayantan than directly off the market. Anasas tend to have high demand. Prices change though.

1. Buy low and Sell High: One of the common strategies out there when it comes to making platinum. This requires patience and knowledge of the items you want to buy and sell. Similar to the term "flipping" in Runescape if that still occurs.
2. Hoard and Sell: what this means is you basically hoard all the items and when you see a price raise, you sell them all in large quantities, not bulk. This is not a buy low or sell high because you are not buying anything.
3. Trading Times: Trading can be time consuming, so what I do is I work on my dojo decoration stuff while waiting for people to contact me via Warframe Market. Or just multi task and do other stuff ex. watch youtube.

So there you have it, a simple guide on making plat. Remember to have patience when it comes to trading and anything new, if tradable, may want to sell since price is high and will lower later. You can always get another one. If you guys got any better ways, comment down below as I will update the guide with your method + crediting you as well. Want more Guides? Check out my Youtube Channel below for more videos. Happy Trading everyone!

http://www.youtube.com/tonnytay60