This method of sampling is called cluster. This method is so hard, there are many things that you need to consider. In this case they need to have experiences for at least 12 months in the estate agent.

The problem here is very time consuming. Based on the story they need to have 10 student for every university. Imagine how many universities they have.

I can suggest random sampling, they can use this. They should asked first the respondents if he/she talked to a estate agent. If he said yes then he is qualified to take the interview, it will also lessen the time