# coding: utf8
from django.db import models
from accounts.models import User, Manufacturer
...
class Zayavka(models.Model):
    """
    Заявка
    """
...
prodavec = models.ForeignKey(User, verbose_name=u'Продавец', blank=True, null=True )
...
 def save(self, *args, **kwargs):
        """
        Если продавец не указан, задаем значение рандомом
        """
        prods = User.objects.filter(groups__name=u'Продавец', region=self.region, ).order_by('?')[:1]
        if not self.prodavec:
            self.prodavec = prods
        
        super(Zayavka, self).save(*args, **kwargs)

User.objects.filter(groups__name=u'Продавец', region=self.region, ).order_by('?')[:1]

>>> s = [1,2,3]
>>> type(s[:1])
<type 'list'>
>>> type(s[0])
<type 'int'>
>>> 

User.objects.filter(groups__name=u'Продавец', region=self.region, ).order_by('?')[:1]

User.objects.filter(groups__name=u'Продавец', region=self.region, ).order_by('?')[0]