Right, so there's essentially two different types of trading.
The first way is the old way, person-to-person. You contact the seller via message, get an invite to their Dojo (you have to have a Dojo with a Trading Post to be able to trade your stuff), then they set up the trade with you.
The second way is kind of a version of what a lot of the Korean and Chinese MMOs have where you can set up your own personal trading stall at Maroo's Bazaar. Go to the Bazaar, see if anyone has what you want, then start interacting with them to set up the trade.
You're limited to a number of trades per day equal to your Mastery Rank, and there's a tax on trades depending on the object you're trading. It's usually a low amount of Credits, unless it's a Primed Mod in which case it's a cool million. Pairs super well with Bladed Rounds but I don't have that one.