Trading is actually pretty straightforward once you understand the core systems (this is by design, to keep the economy player-driven while preventing automation). The main way most players trade is through direct player interaction, where you'll meet up in someone's hideout and use the trade window - always double check the items being offered because this helps prevent scams that sometimes pop up.

For finding specific gear, you'll want to use the official trade site which lets you search for exactly what you need (this is especially helpful when looking for particular stat combinations). The system is built around player interaction, which is why you need to message sellers and arrange meet-ups rather than having an auction house. Interesting to note that you'll need a premium stash tab to list your own items, and your character needs to be level 25+ because this helps maintain economy stability.

Once you complete the campaign, you'll unlock the currency exchange which is really useful for converting different currency types - this system reflects actual market values determined by player trading patterns. Just keep in mind that gold can't be traded between players (it's specifically designed for npc interactions only).

A useful thing to remember is that while trading is a big part of the game's design, you can absolutely play self-found if that's more your style. The loot system is balanced to support both approaches, which gives you nice flexibility in how you want to progress through the game. If you're just starting out, maybe try playing self-found first to learn the item values, then dive into trading once you're comfortable with the basics.