Django 3.0: django-notifications giving error: "Notification.recipient" must be a "User" instance

I am using django-notifications-hq package link of django-notifications-hq and getting error:

Cannot assign "<Seller: john@gmail.com>": "Notification.recipient" must be a "User" instance.

my custom user model:

class User(AbstractBaseUser):
    email = models.EmailField(max_length=255, unique=True)
    name = models.CharField(max_length=255, blank=True, null=True)
    staff = models.BooleanField(default=False)
    admin = models.BooleanField(default=False)
    customer = models.BooleanField(default=False)
    seller = models.BooleanField(default=False)
    timestamp = models.DateTimeField(auto_now_add=True)
    is_active = models.BooleanField(default=True)

my seller model having 1to1 relation with user:

class Seller(models.Model):
    user = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    seller_picture = models.ImageField(upload_to='profile_pic/seller', null=True, blank=True)

my Book model has foreign key seller:

class Book(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    title = models.CharField('Title', max_length=255)
    authors = models.ManyToManyField(Author, related_name='book_written_by')
    seller = models.ForeignKey(Seller, on_delete=models.CASCADE)
    price = models.DecimalField('Price', decimal_places=2, max_digits=10)
    description = models.TextField('Description')

And somewhere in my views after successfully getting books just bought by some user, I want to notify all respective sellers that someone has bought their book:

for book in books:
     notify.send(Seller, recipient=book.seller, verb="User has just bought book named {} priced 
                                                             as {}".format(book.title, book.price))

I know it works fine recipient = request.user but I have different scenario. How can I make book.seller a User instance ?

It's a super simple solution. You're 99% of the way there!

Instead of this:

for book in books:
     notify.send(Seller, recipient=book.seller, verb="User has just bought book named {} priced 
                                                             as {}".format(book.title, book.price))

Try this:

for book in books:
     notify.send(book.seller.user, recipient=book.seller.user, verb="User has just bought book named {} priced 
                                                             as {}".format(book.title, book.price))

The Seller has a property called user but it isn't a user itself.