Business brokers make a living selling businesses but they will only list a business if they think they can make money. Many small transactions are contingent based. Some brokers charge a small retainer to list a business for sale and others do not. There is a lot of time and effort spent by the broker to sell a business. By asking the client for a retainer, it proves that the seller of the business is serious about selling the business and not just shopping it around to come up with a business valuation.
If the selling price is too low, the commission that the broker will get is very small. Many brokers have overhead that they must carry on a monthly basis. If you list a business for sale with a broker and the selling price is say $10,000 and the commission is only $500, many brokers would not be interested in taking on the business to sell. If a broker has no other listings, he may take on the sale but many brokers may look and say that the costs to sell the business exceed the commission therefore they turn down the offer to list the business. If they know someone who is looking for exactly that type of business and they have little work to do, they may accept the assignment.