How does trading work in Warframe for a new player?

Also you need two-factor authentication turned on for your account before trading works, even at MR2. Catches a lot of new people out.

You need Mastery Rank 2 before you can trade at all, and you get as many trades per day as your mastery rank, so they're limited early on. Trades happen at Maroo's Bazaar on Mars or a clan dojo trading post, and each trade costs a credit tax. For prices almost everyone uses the Warframe Market fan site, search the item name. Look at recent sale prices, not the cheapest listing, people list silly low or high all the time. Not getting ripped off: check what's in the trade window before you confirm, and don't sell a full set as loose parts, sets usually go for more. Common parts are worth a few plat at best, most of them only sell for ducats.