If you need a guide to start you off with trading or avoiding scams, here's an article I wrote about it.
At one point or another you might have wondered how you'd get plat for another slot for a new weapon or Warframe. Yes you do get 50 plat at the beginning, but you might have spent that already and still need more.
What I'll be sharing is a way to earn a little plat almost everyday to get you the things you need, or to get you the things you want like that new cape. all you need is time and dedication.
The Daily Grind
These are activities that I suggest you do everyday that will earn you platinum.Syndicates
I suggest that you choose either New Loka, Perrin Sequence and Red Veil or Cephalon Suda, Arbiters of Hexis and Steel Meridian.
In both cases you will be spending time earning affinity with your chosen factions daily. How will you earn from this, well, you can sell either the weapon or the augments (mods) for platinum. Weapons can be sold for 30-40 plat and each augment mod can cost 10-15 plat.
You will only be able to reap the benefits after you hit max rank in a syndicate (this is when you can access the mods and weapons) so be prepared to make those offerings!
Read more about syndicates here.
Raids
If you have the gear, why not go raiding? You can go on raids once a day and you can sell the Arcane you get from the raid. Depending on the arcane you can sell it for 30 plat and up.
Make sure you watch videos and research the raid so you can prepare for it.
Invasions
More specifically, Alad V Coordinates. You need 3 to craft a Key. and Each Key sells for 10-15p.
Other Sources
On top of those daily activities you can also earn platinum via more opportunistic means.
Duck Hunting
Sounds weird, but i find that it's an apt term for jumping into void missions for prime parts. Depending on the part, you can sell it for 1-5 plat. The basic conversion is 10 ducats:1 plat. It's a thing because of Baro, the void trader who gives people access to Primed Mods and Prisma weapons.
The best missions for this are Capture and Exterminate, t1 or t2. If you can get key shares that would easily multiply your potential earnings.
Try to sell it on the days where Baro is present to check for price increases or people who are willing to pay more for Orokin Ducats.
Selling Maxed 10-Rank Mods
Got a lot of credits and r5 Cores? Don't really need to upgrade anything now? Why not get an in-demand 10 rank mod, max it and sell it. Check the market and this guide to see what your prospects are. Depending on the market you can make a lot of plat. The most I earned was for selling a maxed Primed Point Blank for 950 plat (this only cost about 528 r5s and about 1.66 million credits) earning me about 422 plat (r5 to plat is roughly 1r5:1plat)
In order to prevent losses, i suggest that you sell a 10-rank mod you already have and just make another after the sale. This ensures that your mod will be bought vs making something that doesn't sell or for those times when the buyer disappears from one reason or another.
Selling In-Demand Items
These could range from a specific Mod or Prime part. Make sure to watch the market for the trends. As of this writing people are looking for Lifestrike - unranked (20-50 plat) and Ash Prime BP (about 100 plat). Vermilion Storm is also a thing and seems to be very expensive (i can't quote a price though). If you have any of the Event mods that aren't easy to obtain, like the buzz kill set, you can also make a killing on those.
Aside from the high price items, also take note of the prime parts or mods that people need but cost 5p and up. This could be a stable source of income if you of you farm mods or go duck hunting a lot.
The wiki contains the location of most the mods and Prime items (if not all). You can find it here.
Notes:
Most of the things mentioned above can be done at the same time. For example Earning Syndicate affinity or reputation can be done during duck hunts or while you're on a raid.Selling items in the market requires patience. While peddling, watch the market for trends. Knowing the prices will definitely help you earn plat.
Thanks for reading!