You gotta be Mastery Rank 2 at least, then head to a relay or dojo. Super easy once you get the hang of it. Trading posts in dojos are where the real deals go down. Platinum moves fast there.
Make sure to know your prices tho, people will try to shark you. Use Warframe Market.
Only certain items can be traded, like mods, arcanes, rivens. No blueprints or resources mostly.
Always double-check before confirming trades. No backsies in this game.
Tax varies by item too. Some rare stuff gets taxed heavy.
Best way to make plat without swiping the card, fr.