INSERT INTO cart (product_id, quantityCart) VALUES (1, 10) ON DUPLICATE KEY UPDATE quantityCart = quantityCart + 10;
UPDATE products SET quantity = quantity - 10 WHERE product_id = 1;

INSERT INTO cart (product_id, quantityCart) VALUES (2, 15) ON DUPLICATE KEY UPDATE quantityCart = quantityCart + 15;
UPDATE products SET quantity = quantity - 15 WHERE product_id = 2;

INSERT INTO cart (product_id, quantityCart) VALUES (3, 20) ON DUPLICATE KEY UPDATE quantityCart = quantityCart + 20;
UPDATE products SET quantity = quantity - 20 WHERE product_id = 3;

<?php
   if (isset($_POST['addCart']) && $_POST['addCart']=="Add Items to Cart") {
     foreach($_POST['qtyBuy'] as $index=>$value){
       if($value > 0){
        $cartProd_id = $_POST['product_id'][$index];

           $addQuery = "INSERT INTO cart (product_id, quantityCart)
                        VALUES ($cartProd_id, $value)
                        ON DUPLICATE KEY UPDATE quantityCart = quantityCart + $value;";
           $addQuery .= "UPDATE products SET quantity = quantity - $value WHERE product_id = $cartProd_id;";

           $execQuery = mysqli_multi_query($connection, $addQuery);

           echo $addQuery;
        }
     }

  }
?>

<form action="add_sales.php" method="POST">
            <table class="table table-striped table-bordered table-hover results table-fixed table-condensed">
              <thead>
                <tr>
                    <th class="text-center">#</th>
                    <th>Product Name</th>
                    <th>Description</th>
                    <th>Price</th>
                    <th>In Stock</th>
                    <th style="width: 20%">Quantity</th>
                </tr>
                <tr class="warning no-result">
                  <td colspan="8"><i class="fa fa-warning"></i> No Product Found</td>
                </tr>
              </thead>


              <tbody>
              <?php
                $query = "SELECT * FROM products;";
                $exec = mysqli_query($connection, $query);
                $a = 1;
                $b = 1;

                while ($row = mysqli_fetch_array($exec)) {
                  $product_id = $row['product_id'];
                  $product_name = $row['product_name'];
                  $product_price = $row['sell_price'];
                  $description = $row['description'];
                  $product_quantity = $row['quantity'];

               ?>
              <tr>
              <td class="text-center"><?php echo $product_id; ?>
                <input type="hidden" name="product_id[]" value="<?php echo $product_id; ?>">
              </td>
                  <td><?php echo $product_name; ?></td>
                  <td><?php echo $description; ?></td>
                  <td><?php echo $product_price; ?></td>
                  <td><input type="number" value="<?php echo $product_quantity; ?>" id="<?php echo "qtyResult" . $a++; ?>" disabled></td>
                  <td><input type="number" name="qtyBuy[]" id="<?php echo "qtyBuy" . $b++; ?>" onkeyup="updateStock(this, event)"></td>
              </tr>
              <?php } ?>
              </tbody>
          </table>


          </div>
          <div class="form-group">
              <input type="submit" name="addCart" value="Add Items to Cart" class="btn btn-info pull-right">

          </div>
          </form>