Warframe.market is a third-party website that's focused on trading items between gamers in Warframe. It simply acts as an exchange (but the actual trading of items will be done in-game).

Simply browse or search for the item you need to buy or sell using the search bar. To see the prices of others listing the same item.

To get more platinums (in-game currency), I'm trying to sell some mods and prime parts of mine.

It's cool that they also have graphs like when you're trading stocks or cryptocurrencies.