Trading is an activity that involves interaction with other players.
You may enter a trade with another player, chat to them, and add items/gold.
You can also trade using the Auction House can be used to buy or sell all types of weapons (Common, Rare, Epic, Legendary, Mythic, Spirit, Set, & Arcane) which are obtained by other players. Before you buy a weapon, you can look at it's stats and effects then determine what is best for your character/playstyle.