Inventory and Merchant Script problems C#

im having some trouble with my simple inventory and merchant script i want to make it so when the play presses buy item it makes its 2d texture appear in the inventorys GUI window.

Heres the Inventory Script:

 using UnityEngine;
 using System.Collections;
 
 public class InventoryScript : MonoBehaviour {
     public Rect inventory = new Rect(20, 20, 20, 250);
     private BuyItem item;
 
     void Start(){
         item = GetComponent<BuyItem>();
     }
 
 
     void OnGUI() {
         inventory = GUILayout.Window(0, inventory, DoMyWindow, "My Inventory");
     }
     void DoMyWindow(int windowID) {
         item.itemTexture = GUILayout.Box(itemTexture);
         
     }
 }

And heres the BuyItem script:

using UnityEngine; using System.Collections;

public class BuyItem : MonoBehaviour {

 public string itemName = "Sword";
 public Rigidbody item;
 public string itemDesc = "A cheap sword";
 public int itemValue = 10;
 public Transform player;
 private InventoryScript playerInventory;
 public Texture itemTexture;
 
 void Start(){

     playerInventory = GetComponent<InventoryScript>();

 }


 void OnMouseEnter (){
     OnGUI();

 }

 void OnGUI () {
     if(GUI.Button(new Rect(220,220, 150, 100), "Buy " + itemName + " Cost: " + itemValue))
         OnMouseDown();
 }
 
 // Update is called once per frame
 void OnMouseDown () {
     playerInventory.useGUILayout ("itemTex", itemTexture);
 }

}