Selling (and buying) a home is a big deal. Most people don't sell more than two homes in their lifetime, so it's not something they develop expertise in. There are ever-changing laws, ethical practices and practical considerations that you don't want to learn on your own through trial and error or relying on a book or information on the internet. It's just not worth the risk.
Not all real estate professionals are REALTORS. Only licensed real estate agents who are members of the National Association of REALTORS can call themselves REALTORS. REALTORS are committed to treat all parties honestly - they subscribe to a strict code of conduct and are required to maintain a high level of real estate standards. It is to your advantage to work with a REALTOR because of the training and standards this professional designation requires.