GUI related issue

Hey, as a designer, I am pretty new to coding in unity and I decided to make a game on my own to learn javascript.The biggest problem I have now is that the GUI from every "PlacementPoint" is overlapping, only the one in the foreground being usable. Is there a way I can set all my game objects to respond to only one GUI button?
Here's the code:

Code (csharp):

1. #pragma strict
2.  
3. var selectedMaterial;
4.  
5. var material_wood : GameObject;
6. var material_enforcedWood : GameObject;
7.  
8. var defaultSize : Vector2;
9.  
10. function Start ()
11. {
12.     defaultSize = transform.localScale;
13. }
14.  
15. function OnGUI ()                      //Here is my problem I think
16. {
17.     if(GUI.Button(Rect(10, 10, 100, 100), "Wood"))
18.     {    
19.         selectedMaterial = material_wood;
20.         Debug.Log("Wood selecter");
21.     }
22.        
23.     if(GUI.Button(Rect(115, 10, 105, 100), "Enforced Wood"))
24.     {
25.         selectedMaterial = material_enforcedWood;
26.         Debug.Log("Enforced wood selected");
27.     }
28. }
29.  
30. function OnMouseOver ()
31. {
32.     transform.localScale = Vector2 (1.5, 1.5);
33.     if(Input.GetButtonDown("Fire1"))
34.     {
35.         Instantiate(selectedMaterial, transform.position + Vector3(0,0,-0.5) , transform.rotation);
36.     }
37. }
38.  
39. function OnMouseExit ()
40. {
41.     transform.localScale = defaultSize;
42. }
43.  

Thanks in advance.