You do have some unnecessary code, the plugin does not need main method:
* @param args the command line arguments
public static void main(String[] args) {
// TODO code application logic here
Also, you may also want to remove the following in your onCommand method:
Server server = getServer(); // remove this line
Finally, you are calling "event.getPlayer()" even though you've already made a reference to player at the start of the onCommand method "Player player = event.getPlayer();".
Once a reference has been made you can keep using it, don't keep calling the event.getPlayer method. You've kind of chopped and changed in there.
player.playGameSound("trade_success"); //correct use
event.getPlayer().setInvisible(false); //should just be player.setInvisible(false);
