Both a persuasive request and a sales message try to convince someone to do something. A persuasive request can be written to an individual or to a large group pf people. It can be used to ask someone to purchase something, for example a child could write a persuasive letter to a parent requesting the latest piece of technology. It could also be a person or group of people trying to change the behavior or actions of others. An example is Disney's Friends for Change program in which the Disney Channel actors/actresses are asking kids to change their behaviors to help the environment.
A sales message has one primary purpose. It is designed to encourage consumers to purchase the product being advertised.