The best equipment in the game can always be found by adventuring. This means that sometimes you’ll find a really good item that you can’t use. But there is probably someone else who would love to trade you something for it.
When you find someone that you want to trade with, left-click the icon above their head. This will open a small menu with several options like Trade, Whisper, and Invite. Choosing Trade will send them an offer to trade with you which they can accept or decline. If they accept, a secured trading window will open where you can both put the items you’re interested in swapping.