In Warframe, clans are basically player-run guilds that you join to find friends to play with and access all sorts of handy features-one of the them being access to a trading post. You can buy platinum for real money or sell items to make some. Most players prefer to trade their items for platinum (often written as 'p'), Warframe's in-game currency.
We'll cover the clan dojo method first since it's the most common way of trading. Trading in Warframe happens in one of two locations: A clan dojo or Maroo's Bazaar.