[TelaFirma]

Ok, here is a quick little mod that will do that for you. This will give you a checkbox in Admin (in Appearance Options, right under the Display Order dropdown) that will allow you to check if you want the listing in revere order or not, and it will also change the "Default" order method to ProductID.

Open products.php and look for the lines:

if(!isset($sort))
	$sort = $config["Appearance"]["products_order"];
if(!isset($sort_direction))

Right under this add this code:

{
if ($config["Appearance"]["product_order_reversed"] == "Y") {
    $sort_direction = 1;
} else {
    $sort_direction = 0;}
}

Open /include/search.php and look for this block of code:

		# Sort the search results...

		$direction = ($data["sort_direction"] ? "DESC" : "ASC");
		switch ($data["sort_field"]) {
			case "productcode":
				$sort_string = "$sql_tbl[products].productcode $direction";
				break;
			case "title":
				$sort_string = "$sql_tbl[products].product $direction";
				break;
			case "orderby":
				$sort_string = "$sql_tbl[products_categories].orderby $direction";
				break;
			case "quantity":
				$sort_string = "$sql_tbl[products].avail $direction";
				break;
			case "price":
				if (!empty($active_modules["Special_Offers"]) && !empty($search_data["products"]["show_special_prices"])) {
					$sort_string = "x_special_price $direction, price $direction";
				}
				else {
					$sort_string = "price $direction";
				}
				break;
			default:
				$sort_string = "$sql_tbl[products].product";
		}

and replace the line

				$sort_string = "$sql_tbl[products].product";

with

				$sort_string = "$sql_tbl[products].productid";

Run this SQL patch in the Patch/Upgrade section

INSERT INTO `xcart_config` VALUES ('product_order_reversed', 'List products in reversed order', 'Y', 'Appearance', 100, 'checkbox', 'Y');