class Account < ActiveRecord::Base
  has_many :users, :as => :account, :dependent => :destroy
end

class Buyer < Account
  set_table_name :buyers
end

class Seller < Account
  set_table_name :sellers
end

class User < ActiveRecord::Base
  belongs_to :account, :polymorphic => true
end

create_table :users do |t|
  t.references :account, :polymorphic => true
  t.string :name
end

create_table :buyers do |t|
  t.string :company
end

create_table :sellers do |t|
  t.string :organization
end

Loading development environment (Rails 2.0.2)
>> b = Buyer.create(:company => "FooCo")
=> #
>> b.users << User.new(:name => "Joe User")
=> [#]
>> s = Seller.create(:organization => "BarOrg")
=> #
>> s.users << User.new(:name => "Sam OtherUser")
=> [#]
>> User.find(:first).account
=> #
>> User.find(:first).account.users
=> [#]
>> User.find(:first).account.class
=> Buyer(id: integer, company: string)
>>