Question on Milton Friedman on Government Certification

I am reading Milton Friedman's "Capitalism and Freedom". In his discussion of possible justifications for government certification of professions, he offers the paragraph below, which I simply do not understand. Can anyone figure it out?

One can however argue that in some cases, or perhaps even in many, voluntary certification will not be carried as far as individuals would be willing to pay for carrying it because of the difficulty of keeping the certification confidential. The issue is essentially the one involved in patents and copyrights, namely, whether individuals are in a position to capture the value of the services that they render to others. If I go into the business of certifying people, there may be no efficient way in which I can require you to pay for my certification. If I sell my certification information to one person, how can I keep him from passing it on to others?

If a private venture fails it's closed down. If a government venture fails it's expanded. M Friedman