It seems to me that the problem is the way you are handling the validations and showing the resulting error message. Currently you are only validating the posted element hbCart_1_Quantity_Add. If you were to try and add more that was available for this item it will trigger the server side validation and cause the error message to show.
To make it show the message only shows for a particular product you will first need a way to identify which item on the page was trying to be added. You could store the value of the hbCart_1_ID_Add posted element in a session variable. Then when the validation for the quantity fails you can have a check around the validation show if error message that also checks to see that the current item id is the value held in the session variable.
If you had it setup like this then you would be able to make sure that the message only showed for the id of the item that was posted. For example if you stored the submitted item id in a session variable called itemID you could add an if statement around the validation show if like this:
<?php if ($_SESSION['itemID'] == $row_rsBooks["id_inventory"]){ ?>
//validation show if code here
<?php } ?>
