What's the best way to make platinum? I keep hitting walls when my inventory gets full and when I have no more warframes and weapons to rank up.
I bought plat with real money once, but I'd rather not have to keep doing that if I can make money in-game.
One of the toughest things to do that will always net you a ton of platinum is to play the game right when a new prime set comes out. Farm and sell
all of the new things you get. Don't try to hold onto one for yourself, just dump it into the market. If it's a particularly beloved Warframe or weapon, hold up to a set.
On PC, I've found that the first week has people frothing to get the new things. Common parts go for at least 50p, rares upwards of 500p. A full set might go for a thousand or more. Things might be more relaxed in price on PS4 but I'd be willing to bet that the desire for the new thing is still there.
But it sucks because you also want the new thing right there, right now. Just keep in mind that by the end of a new cycle, you can usually buy entire set of what you need for cheap.
Sell your veiled Rivens. On PC I can get 50p for moderate or easy challenges, especially for the Shotgun rivens. Sure you might roll it into something good, but you might also roll something not even worth the endo it melts down into. On that note, you can also make a small killing on fully stocked Ayatans.
Also don't be afraid of making small sales. Life strike for 10p, the syndicate mods for cheap, etc. Use up your trades for the day. People become obsessed with the one big payout trade when they could be making just as much (if not more) with many easy sales. Most of the Corrupted Mods (the ones that come from Orokin Derelict Vaults) sell pretty good. Rage is another easy sale off the top of my head.
Be kind and courteous to those you trade with. Be willing to haggle a bit, and know how to work the system for your favor. Want 30p? Start the selling price at 35 or 40. When you drop your price down to what the other person wants, it ends up making them feel like they got a deal, and you're getting the actual amount you want. Just don't set the price dumbly high, otherwise you won't get any bites.
I wish Warframe.Market would support a PC/PS4 split. It's a great resource for figuring out the price or even making sales.