In this model, goods usually go from a producer to a wholesaler to a retailer to a consumer. In a free market system, each of those involved has to take a profit to stay in business. That means that as the product moves to the next step in the channel, a percentage is added to the price. The channel here generally runs from a provider, through a broker, to the consumer. Examples would be real estate and insurance sales.
Direct sales can be a tough way to make a living.
The broker is generally paid only on personal sales. Some brokers add a level by using sales agents, splitting the commissions according to an agreed-on percentage. There are no residuals. There's an inside joke among salesmen that the worst thing about making a sale is that you just lost your best prospect. Among real estate people, the joke is that as soon as you sell a house, you're out of a job.