Display In-Stock Seller/Vendor on top of product view page

You can order the position of seller in your collection by modifying your both the seller collection like below.

As you have already filtered the Seller list by quantity having more than 0 value.

By ordering Price and Qty both you will get seller having less price at first place and if both the seller having same price then it will consider seller having more number of qty for that particular product as first place.

You can apply these to both your collection.

public function sellerUsedProductList() {
    $productid=Mage::registry('current_product')->getId();
    $collection = Mage::getModel('mpassignproduct/mpassignproduct')->getCollection()
                    ->addFieldToFilter('product_id',array('eq'=>$productid))
                    ->addFieldToFilter('qty',array('gt'=>0))
                    ->addFieldToFilter('product_condition',array('eq'=>'used'))
                    ->addFieldToFilter('flag',array('eq'=>'1'));
 //    $collection->setOrder("price",ASC); your order by commented
          $collection->getSelect()->order("price ASC")->order("qty DESC");
    return $collection;
}

Let me know if you still have any problem.