If you don't like the email feature, here's how to turn it off:
Open Sources/shop.php
Find: (lines 785 - 789)
//Send message to original owner
$to = $row_item['emailAddress'];
$subject = "Your trade of '{$row_item['name']}' item";
$message = "Congratulations! {$row_user['realName']} has purchased your '{$row_item['name']}' item from you for ".formatMoney($row_item['tradecost']).".\r\n\r\nHave a good day,\r\n --Forum Management";
mail($to, $subject, $message);
See the mail($to, $subject, $message) line? Just put a // in front of it (eg.:
//mail($to, $subject, $message);
This will comment out that line, and stop the email from being sent.