It is actually quite hard not to run a successful employment agency but it all depends on the ability to sell your service to the employers in any given area.
Specialisation is often a key to success. We went for the Tutoring/Teaching speciality as there were others, better established, dealing with Drivers, Typists, Clerical etc in the area when we arrived. Health prevented me from building on the excellent first year's trading and SWWLTBO is no salesman so we decided to sell the business. Had we managed to hang on for two more years we would have made a lot out of it but it is hard to run a business from a hospital bed.
Of course "Temping" agencies do enjoy economic conditions such as those currently prevailing in the UK as many businesses prefer not to invest in staff but would rather limp along using temps as the cash flow is predictable and you can always tell the agency "Not this week thanks" whereas you can't always lay off directly employed staff that quickly.