Trading is one of the basic features of Perfect World. You can request a trade from other players. To do this right-click on a player's name or a player's avatar to bring up an actions menu. There you will find a variety of options including the "Trade" button.
The other person will get a notification, and can accept or deny your request to trade. If they accept, a new box will open that will have what they are offering on top, and slots for the items you are offering on the bottom. Then all you do is drag and drop. To trade coins, you click on the coins icon in the trade window.
For selecting stacked items that you want to split up, press the ALT key and drag item to either trade slot or an available slot in your inventory.