Сортировка товаров по популярности (количеству добавлений в корзину)

// событие вызывается перед добавлением товара к корзину. на вход передаются параметры товара. в случае если возвращает false, товар в корзину добавлен не будет. 
// http://dev.1c-bitrix.ru/api_help/sale/sale_events.php
AddEventHandler("sale", "OnBeforeBasketAdd", Array("YenBasket", "BeforeBasketAdd"));

class YenBasket
{
   function BeforeBasketAdd($arFields)
   {
      // Подключаем модуль
      if(CModule::IncludeModule("iblock"))
      {
         // Получаем элемент
         $res = CIBlockElement::GetByID($arFields["PRODUCT_ID"]);
         if($ar_res = $res->GetNextElement())
         {
            // Получаем значение свойства-счетчика
            $COUNT_TO_BASKET = $ar_res->GetProperty("COUNT_TO_BASKET");

            // Увеличиваем счетчик на единицу и сохраняем значение в свойстве
            CIBlockElement::SetPropertyValueCode($arFields["PRODUCT_ID"], "COUNT_TO_BASKET", intval($COUNT_TO_BASKET["VALUE"])+1);
         }
      }
   }
}

<div class="sort">Сортировать по:
<?if($_REQUEST["sort"]==hit):?><span>популярности</span><?else:?>
<a href="<?=$APPLICATION->GetCurPageParam("sort=hit", array("sort"))?>" title="актуальности">популярности</a><?endif;?>
</div>

<?
if($_REQUEST["sort"]=="hit")
{
   $sort="PROPERTY_COUNT_TO_BASKET";
   $by="DESC";
}
else
{
   $sort="NAME";
   $by="DESC";
}
?>

<?$APPLICATION->IncludeComponent("bitrix:catalog.section", ".default", array(
   "IBLOCK_TYPE" => "catalog",
   "IBLOCK_ID" => "3",
   "SECTION_ID" => $_REQUEST["SECTION_ID"],
   "SECTION_CODE" => "",
   "SECTION_USER_FIELDS" => array(
      0 => "",
      1 => "",
   ),
   "ELEMENT_SORT_FIELD" => $sort,
   "ELEMENT_SORT_ORDER" => $by,
   "FILTER_NAME" => "arrFilter",
   "INCLUDE_SUBSECTIONS" => "Y",
   "SHOW_ALL_WO_SECTION" => "Y",
   "PAGE_ELEMENT_COUNT" => "100",
   "LINE_ELEMENT_COUNT" => "3",
   "PROPERTY_CODE" => array(
      0 => "MARKER",
      1 => "",
   ),
   "SECTION_URL" => "",
   "DETAIL_URL" => "",
   "BASKET_URL" => "/personal/basket.php",
   "ACTION_VARIABLE" => "action",
   "PRODUCT_ID_VARIABLE" => "id",
   "PRODUCT_QUANTITY_VARIABLE" => "quantity",
   "PRODUCT_PROPS_VARIABLE" => "prop",
   "SECTION_ID_VARIABLE" => "SECTION_ID",
   "AJAX_MODE" => "N",
   "AJAX_OPTION_SHADOW" => "Y",
   "AJAX_OPTION_JUMP" => "N",
   "AJAX_OPTION_STYLE" => "Y",
   "AJAX_OPTION_HISTORY" => "N",
   "CACHE_TYPE" => "A",
   "CACHE_TIME" => "36000000",
   "CACHE_GROUPS" => "Y",
   "META_KEYWORDS" => "-",
   "META_DESCRIPTION" => "-",
   "BROWSER_TITLE" => "-",
   "ADD_SECTIONS_CHAIN" => "N",
   "DISPLAY_COMPARE" => "N",
   "SET_TITLE" => "N",
   "SET_STATUS_404" => "N",
   "CACHE_FILTER" => "N",
   "PRICE_CODE" => array(
      0 => "BASE",
   ),
   "USE_PRICE_COUNT" => "N",
   "SHOW_PRICE_COUNT" => "1",
   "PRICE_VAT_INCLUDE" => "Y",
   "PRODUCT_PROPERTIES" => array(
   ),
   "USE_PRODUCT_QUANTITY" => "N",
   "DISPLAY_TOP_PAGER" => "Y",
   "DISPLAY_BOTTOM_PAGER" => "Y",
   "PAGER_TITLE" => "Товары",
   "PAGER_SHOW_ALWAYS" => "N",
   "PAGER_TEMPLATE" => "",
   "PAGER_DESC_NUMBERING" => "N",
   "PAGER_DESC_NUMBERING_CACHE_TIME" => "36000",
   "PAGER_SHOW_ALL" => "N",
   "AJAX_OPTION_ADDITIONAL" => ""
   ),
   false
);?>