$this->seller
is not an object because $this->getUser()->getGuardUser($seller_id);
don't accept any parameters, it only retrieves current user. Look at that method definition at sfGuardSecurityUser
.
You have to do it like:
Doctrine: $this->seller = $this->car->getSeller();
Propel: $this->seller = SfGuardUserPeer::retrieveByPK($seller_id);