The if statement that you have now is setup like this:
<?php if ($row_rsInventory['quantity'] = 0) { // Show if recordset empty ?>
<?php echo $row_rsInventory['backordermsg']; ?>
<?php } // Show if recordset empty ?>
To make it so this message will only show if there is not at least one item available it should look like this:
<?php if ($row_rsInventory['quantity'] < 1) { // Show if recordset empty ?>
<?php echo $row_rsInventory['backordermsg']; ?>
<?php } // Show if recordset empty ?>
This is saying that if the current row in the rsInventory recordset for the quantity column is less than 1, then display the value backordermsg from the rsInventory recordset for the current row. If you do not see the message then it would indicate that the value from this column is 1 or greater. If you have any further problems with this you should check your db for the specific record involved to see what the exact values are for these columns.