Why isn't the window draggable?

I want to make this window draggable, why isn’t it dragging?
Heres the code:

public class ButtonsAtTop : MonoBehaviour {

	public Texture LogOutTexture;
	public Texture Shop;
	public Texture Inventory;
	public Texture Friends;
	public Texture Chat;
	public Texture CharacterCustom;
	public Rect WindowSize = new Rect (700f,700f,1000f,1000f);
	bool displayWindowForShop=false;

	void OnGUI()
	{

		if (GUI.Button (new Rect (Screen.width - 50, 0, 50, 50), LogOutTexture)) 
		{
			PhotonNetwork.Disconnect();

		}
		if (GUI.Button (new Rect (Screen.width - 100, 0, 50, 50), Shop)) 
		{
		displayWindowForShop=!displayWindowForShop;
		}
		if(GUI.Button (new Rect (Screen.width - 150, 0, 50, 50),Inventory))
		{
			
		}
		if(GUI.Button (new Rect (Screen.width - 200, 0, 50, 50),Friends))
		{
			
		}
		if(GUI.Button (new Rect (Screen.width - 250, 0, 50, 50),Chat))
		{
			
		}
		if(GUI.Button (new Rect (Screen.width - 300, 0, 50, 50),CharacterCustom))
		{
			
		}
		if (displayWindowForShop) 
		{
			GUI.Window(0, new Rect(Screen.width-750,(Screen.height/2)-250,500,500), ShopWindow, "Shop Window");
		}
	
	}

	void ShopWindow(int id)
	{

		GUI.DragWindow ();
	}

	void OnDisconnectedFromPhoton()
	{
		Application.LoadLevel ("SciFi Level");
	}

}

This is because you never update the rect of the window. Try using the following:

using UnityEngine;

public class ButtonsAtTop : MonoBehaviour {
    public Rect showWindowRect;
    bool displayWindowForShop=false;

    void Start() {
        displayWindowForShop=true;
        showWindowRect=new Rect(Screen.width-750, (Screen.height/2)-250, 500, 500);
    }

    void OnGUI() {
        if(displayWindowForShop) {
            showWindowRect=GUI.Window(0, showWindowRect, ShopWindow, "Shop Window");
        }

    }

    void ShopWindow(int id) {
        GUI.DragWindow();
    }
}