Dejte return $verkopen; po vašem foreach.
public function verkopenLijst() {
$query = $this->db->query("SELECT * FROM `shop_order_product`");
$verkopen = array();
if($query->num_rows > 0){
foreach($query->rows as $result){
$verkopen[] = array(
'name' => $result['name'],
'model' => $result['model'],
'quantity' => $result['quantity'],
'price' => $result['price'],
'total' => $result['total'],
'tax' => $result['tax']);
}
}
return $verkopen;
}