This is actually a very good question.
Domain brokers can help you buy or sell a domain. For example, if the domain you want is very valuable and already purchased, domain brokers can help negotiate on your behalf and try to bring down the asking price. However, I have never heard of anyone really hiring brokers to buy domains.
In general, brokers are hired to sell your domain name(s) as they have the tools and the reach far better than someone who has never sold a domain name before.
I suggest contacting the owner of the domain name you want yourself without any outside help. Start by asking what price they want for the domain and it will help you understand whether further discussions are even worth your time.
Best,
Artashes