If they want to sell the item they can do one of two things
They can either attach the item to an email and send it to you with a gold cost that you pay in order to claim it
Or they stand near to you, hold the interaction button that appears when you are near someone and select trade from the options wheel, you press the button that appears in order to confirm that you want to trade, they will put the item in the screen that appears, you enter the agreed cost and then you both press E. You can swap items that are not bound to you the same way.