OnTriggerStay and OnTriggerEnter won't react when the player enters them.

Questions & Answers
1

I have a script which checks if the player has entered the trigger and if it is then ask if he wants to buy ammo, the strange thing is - the trigger simply doesn’t work while a similar method works for zombies.

Here is the code:

var Bought : boolean = false;
var LAAS : boolean = false;

function OnTiggerStay (collision : Collider) {
	if(collision.tag == "Player") {
		LAAS = true;
	}
}

function OnTriggerExit (collision : Collider) {
	if(collision.tag == "Player") {
		LAAS = false;
	}
}

function Update () {
	if(LAAS == true) {
		if(Input.GetKey(KeyCode.E)) {
		}
	}
}

function OnGUI () {
	if (LAAS == true) {
		GUI.Label(Rect (10, 50, 300, 20), "Press E to buy 1 mag for 20 points");
	}
}
2

You have a trigger Enter & Exit.

Now how big is the trigger?

Now with your code, you have not signified the purpose to happen when the player hits the ‘e’ button. You might have to look into it.
Now - Lets consider a scenario which I believe is more relavent to yours.

I am assuming that when the player is inside the collider (which is a trigger), the player has the ability buy stuff. Hence why not make the code like this.

function OnTriggerStay (collision : Collider) {
    if(collision.tag == "Player")
 {
      if(Input.GetKey(KeyCode.E))]
  {
       // add your relevant code here. 
       }
    }
}

Let me know, if it works.

Later.