is it possible to display sold out image on auctions once item sells in ZC store

Hi, a lot of the items I sell on my ZenCart store I also list on third party sites such as eBay.

I would like to know if it's possible to display the 'sold out' image on those third-party site listings if the items sells out in my ZenCart store first. That way customers will not be angry if they buy the item and later find out it sold out in our regular store.

Thanks.

Using the Developers Tool Kit in ZenCart, I think I found the function that makes the sold out image display.

<?php
    case ($button_check->fields['products_quantity'] <= 0 and SHOW_PRODUCTS_SOLD_OUT_IMAGE == '1'):
      if ($_GET['main_page'] == zen_get_info_page($product_id)) {
        $return_button = zen_image_button(BUTTON_IMAGE_SOLD_OUT, BUTTON_SOLD_OUT_ALT);
      }
?>

I put that on a file named stockcheck.php and put it on ZC's root directory, and then make the reference to it with
<img src="http://www.domain.com/stockcheck.php?product_id=123">

I'm obviously doing something wrong, expected since I don't know any php:), as this doesn't work.

Can someone please tell me what I need to do to make this work. I'm willing to pay for your time to work on this code.

Thanks.