K Stuck on yhis script

[Rave_J]

kk im working on this shopping system for my server
pretty much what u do is click on the npc the gui pops up
and u hit buy or close

now i got the opening the gui to display when u click but idk how to make it where u can click the buy to buy the item or what can someone help me ?
I have this so far ?

PHP Code:

//#CLIENTSIDE
function onCreated(){
destroy();
}
function onActionLeftMouse(){
new GuiBitmapCtrl("Shopping-Menu") {
  x = 10;
  y = 10;
  widith = 160;
  height = 80;
  bitmap = "shopping_gui.png";
}