If you sell your business yourself, you will save commission otherwise payable to the business broker. You will be able to control who knows about your business for sale, the amount of information that you provide to them therefore fewer people may be aware that your business is for sale. You may know if a person enquiring is a competitor or not therefore you can decide not to provide them information or determine that you should be a premium because a purchase of your business may be synergistic with their business.
If you use a business broker, you are using the expertise of a person who has sold many businesses. They have many contacts of buyers and they know what information should be prepared in an executive summary and what information should be given out. Hopefully, the business broker can determine what is a realistic price for your business and hopefully can get a higher price for the business and sell it faster then you can. If they can get a higher price, it may more than make up the commission that you will end up paying the broker.