Me gustaría dejar de deshabilitar el botón de enviar una vez que se complete la validación y se envíe el formulario, por lo que no tendré varias inserciones en la base de datos. No he actualizado la publicación con la validación completa del formulario, porque la mayoría de ustedes dice que eso está mal, pero no lo creo.

Aquí está el envío:

<input class="button2" style="border-right:none; font-size:13px;" name="List Item" id="submit" type="submit" value="List Item" />

Y a continuación la validación de jQuery:

   <script src="http://jquery.bassistance.de/validate/jquery.validate.js"></script>
<script src="http://jquery.bassistance.de/validate/additional-methods.js"></script>   

<script>
$( "#UploadForm" ).validate({

  errorLabelContainer: "#messageBox",
  wrapper: "td",

  rules: {  

   <?php if ($type == 'Aution' || $type == 'Both') {  ?> 
    auction_price: {
    required: true,
    number: true,
    min: 1.00
    }, 
    auction_reserve_price: {
    number: true,
    range: [1.00, 500000.00]
    },
    <?php } ?>

    <?php if ($type == 'BuyItNow' || $type == 'Both') {  ?> 
    auction_bin_price: {
    required: true,
    number: true,
    range: [1.00, 500000.00]

    }, <?php } ?>


    Country: {
        required: true
    },

    select3: {
        required: true
    },
    select2: {
        required: true
    },
    auction_postage_type: {
        required: true

    },
        auction_postage_type_price: {
        number: true,
        range: [0.00, 500000.00]
    },

    auction_int_postage_type_price: {
        number: true,
        range: [1.00, 500000.00]
    },

     Europe1: {
        number: true,
        range: [1.00, 500000.00]
    },
     Russia1: {
        number: true,
        range: [1.00, 500000.00]
    },
     NorthAmerica1: {
        number: true,
        range: [1.00, 500000.00]
    },
     Australia1: {
        number: true,
        range: [1.00, 500000.00]
    },
     SouthAmerica1: {
        number: true,
        range: [1.00, 500000.00]
    },
     Africa1: {
        number: true,
        range: [1.00, 500000.00]
    },
     Asia1: {
        number: true,
        range: [1.00, 500000.00]
    },
     MiddleEast1: {
        number: true,
        range: [1.00, 500000.00]
    },


    <?php if ($type == 'BuyItNow') {  ?> 
    auction_item_quantity: {
        required: true,
        digits: true,
        range: [1, 99]
    },
    <?php } ?>

    auction_description: {
    required: true
    },
    auction_int_postage_type: {
    required: true
    },
    listing_type: {
    required: true
    }
  },
  messages: {
    auction_price: "Please Enter Auction Start Price - In Decimal Format, minimum value 1.00",
    auction_reserve_price: "Please Enter Auction Reserve Price - In Decimal Format 0.00",
    auction_bin_price: "Please Enter Buy It Now Price - In Decimal Format, minimum value 1.00",
    select3: "Please Select Main Category",
    select2: "Please Select Sub Category",
    auction_item_quantity: "Please Enter Quentity - min 1 - max 99",
    auction_description: "Please Enter Auction Description -  Min 10, Max 1000 characters",
    listing_type: "Please Select Listing Type",
    auction_postage_type: "Please Select Postage Type",
    auction_int_postage_type_price: "Please Enter International Postage Cost In Decimal Format, minimum value 1.00",
    auction_postage_type_price: "Please Enter Postage Cost In Decimal Format",
    auction_int_postage_type: "Please Select",
    Country: "Please Select Country",

  },
  submitHandler:function(form){
       $('#submit').attr('disable',true).css('pointer-events','none');
  }


});
</script>