One day, dad came home, crushed. The engineer, who had the briefcase, had skipped town with it. Without a contract, and without another prototype, there was not much dad could do. Now, two decades later, it is obvious that the auto-dialer made that engineer very rich. Well, the very nature of the capitalistic game is that many different people will be in the same line of business and will compete for the same customers, and they do so without getting anyone's permission.
Competition is the name of the game.
So the rule is that you normally don't need anyone's permission to start a business and enter into the fray. Some companies have territories . You may choose to buy and sell a product with a company that has assigned, for a fee, certain geographical territories to its sales force. Normally, such territories are exclusive to that salesperson. For example, Larmeister Butter may give exclusive rights to sell its butter in California to one of its people, but obviously that does not limit your right to sell other brands of butter.