So, when someone buys an item, you want an email?
In Sources/shop/Shop-Buy.php, find:
// Tell them that they purchased it
$context['shop_buy_message'] = sprintf($txt['shop_bought_item'], $row['name']);
Add above:
// Modified as per http://www.daniel15.com/forum/index.php/topic,784.0.html - Send an email on purchase of an item
// We need the sendmail() function
global $sourcedir;
require_once($sourcedir . '/Subs-Post.php');
// Who are we sending to?
$to = 'i_like_spam@daniel15.com';
// Subject of the email
$subject = '"' . $row['name'] . '" item bought';
// Actual message
$message = 'The user ' . $context['user']['name'] . ' has purchased a ' . $row['name'] . 'item.';
// Send it!
sendmail($to, $subject, $message);
Change the email address, subject and message to suit your needs. Even if you don't know PHP, you should be able to see what the code is doing
($row['name'] is the name of the item).