"Realtor" is a trademarked term which indicates that a person is a member of the National Association of Realtors. This is the org which the parent commenter is calling a monopoly.
https://en.wiktionary.org/wiki/realtor
https://en.wikipedia.org/wiki/National_Association_of_Realto...
This doesn't even make sense. 'Realtors" is plural, and there are many, which is incompatible with monopolies. Realtors have a monopoly on their trade only because anyone competing with them becomes a realtor, by definition.
It's like complaining about the airlines' monopoly in air passenger transport.