Your code that you're showing us doesn't make any sense.
1. You create a GUI somewhere
2. You call setItem somewhere
3. You're using a client flag for gold
If you're creating a GUI in the Shop weapon, add a public function to make the window visible and populate it with the right values instead of using setItem in your level npc.
Weapon: -ShopWindow
http://pastebin.graalcenter.org/read...d=834421076809
Your class:
http://pastebin.graalcenter.org/read...d=040953102775
All I did was put your code in the right places and change a few things that cbk suggested.