In general, professionalism refers to a particular way of doing things,
including the way that a professional communicates with prospective clients.
Briefly, it's about respect, focus, and dignity.
Professionalism is about the quality of the interaction. One person with established skills offers you specialty services for pay while maintaining their self-respect and respecting the dignity of you, the prospective client (as well as the dignity of competing service providers).
A "true professional" values their own time and yours, too. They are focused on quickly determining if you are a good match for their priorities and skills. They will ask questions that are designed to quickly sort good fits from bad fits.
If they suspect that there is not a good fit between you and them, then they will quickly move toward ending the interaction. Further, they respect that even if there is not a good match between you and them, you might know someone who would want to hire them.
If they interrupt you, they will be direct about it. They are not focused on being labeled as polite, but simply on quickly determining if you are a good match for their services.
They are clear on what they are willing and able to do. If they can provide what you need in the way you need it, they want to quickly identify that. If not, then they want to be clear to you that there is not a good fit.