Available to all players once they begin their character.
To trade other players, you can click the other player's name on the side of your screen, selecting the trade button will send the other player a request to trade, when they accept the trade request, a trading menu will pop open, where you can select which item(s) you'd like to trade to the other player.(Don't forget to be observant to any scamming that may happen.)
Players also often organise trades through the discord.
The Market portal can be found in the nexus, though you must meet the requirements.
Players can use the Market to purchase other player items or list their own items for sale.