Business brokers are usually hired by the seller of the business. Sometimes, they are hired for a specific search to find a certain business profile that a buyer would like to find and they are relying on the business broker to find the perfect fit. Sometimes they are looking to acquire a competitor but do not want to reveal their identity because the price my increase significantly.
More typically, the business broker is hired by the seller of the business. Often, they are paid a commission based on the selling price. Some brokers do receive a portion of their commission up front in the form of a non refundable retainer.
Business brokers do not work like real estate agents, often they do not share their business for sale listings with other business brokers and they only sell businesses that their firm office represents.
There is no central listings service for businesses for sale. Some states/provinces have a multiple listing service for businesses for sale similar to the residential real estate market. The majority of businesses are not sold through this system. Even if there is this service, brokers often only show their own listings.