booking.rb

# app/models/booking.rb

class Booking < AR::Base
  # ...
end

# app/models/booking/create.rb

class Booking::Create
  def initialize(attributes)
    @booking = Booking.new(attributes)
  end

  def call
     @booking.tap do |booking|
       if booking.save
         # inform seller
         # inform buyer
       end
     end
  end
end

# app/models/booking/cancel.rb

class Booking::Cancel
  def initialize(booking_id)
    @booking = Booking.find(booking_id).extend(Cancelable)
  end

  def call
    @booking.tap do |booking|
      if booking.cancel!
        # inform buyer
      end
    end
  end

  module Cancelable
    extend ActiveSupport::Concern

    included do
      before_validation :guard_cancelation
    end

    def cancel!
      update_attributes(:status => 'canceled')
    end

    private

    def guard_cancelation
      errors.add(:base, 'can not cancel paid booking') if paid?
    end
  end
end