It partly depends on scale. At the individual freelancer level it is hard to think about or justify paying a salesperson to sell your services. It often becomes clear when you have grown your freelance business into a small consultancy with 4 or 5 consultants that you need to keep busy. Now it becomes clear that what you need is a salesperson whom you pay and whose expertise and interests are aligned with yours.

Another thing to consider is mutual commission arrangements with consultants whose skills and services are complementary to your own. I have this arrangement with a friend 10% commission on referred projects.

It hasn't been a great steady source but a project here and there comes my way.