User model inherit Seller model or vice versa?

[Brandon]

So all Sellers are Users, Users can be Sellers in this marketplace model.

Should Seller be on top and inherit to User or the other way round?

I found it weird when doing a Session login, I've to call Seller.has_sold? even though all they are all Users to begin with, but I can't call User.has_sold? since the method is not in User.rb.