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.
Both sales messages and persuasive requests are intended to influence the behaviour of someone who is under no obligation or compulsion to act in the way desired by the persuader or the seller. The main difference is that the sales message persuades by convincing the prospect that it is in his or her interest to buy. A persuasive message may also persuade by emphasising the benefit to the person being persuaded. However, it may appeal to other kind of motivations also. For example, a persuasive request for donation to a social service organization may appeal to the altruistic feeling of the person rather than the motive of personal profit or gain.