While most contractors or employees will have a set pay rate for their job or project, many sales positions offer a commission based pay structure. In a commission based position, the salesman will earn a percentage of revenue from whatever they sell. You may be able to hire contractors on a purely commission basis, meaning they only get paid when they make sales for your company. A more common situation is an employee with a low base salary, with the opportunity to earn more in commissions based on their performance. A common example of this is car salesmen at dealerships. They earn a smaller than usual salary, but make it up by selling more cars. 

Commission based sales reps can also be a good way to get the word out about your business in the beginning, specifically if you have a local business or are selling a high-ticket product.