To trade with other players, click on the player, then click on the name tag.
A menu will appear, select 'Player Trading'.
Drag the items you wish to trade from your bag into the trade window.
Press Ready to confirm the transaction. Your trade window will grow blue.
When both players are ready the transaction will be complete.
NOTE: Always double check if the items your trading (yourself and the other player) are correct before you confirm. Don't be Scammed!