Hope you're doing well.
$listingsByType contains the number of new/used listings for a product and default_listings.php layout shows the products for new/used listings.
Regarding sellers in default_sellers.php, you would have to code the sorting on components/com_sellacious/models/product.php
Inside the $sellerUids loop, put following after line 692:
$obj->sales_price = $obj->price->sales_price;
And put this after foreach loop ends after line 699:
$sellers = ArrayHelper::sortObjects($sellers, 'sales_price');
This will sort sellers by price in ascending order.
Hope this helps.