I have been an agent in SC, NC, and now TN, but I assume the Authorization to Show forms are similar. Based on that, it is very typical for a real estate agent with a buyer to ask for commission in order to bring in their buyer. Typically, in the areas I have worked, you are paying about half of what you would be paying if you listed your property with an agent. Keep in mind, however, that the agent is representing the buyer's interests and not yours.
I would highly suggest that you become familiar with real estate contracts if you are selling by owner. When someone makes an offer on your property, whether through an agent or as an individual, you need to understand the importance of how it is written in order to protect yourself. Also, please take precautionary measures when showing your property. I would show only by appointment, and make sure the interested buyers are not looking for owner financing if you are not willing to hold the mortgage.
I hope this is helpful and good luck!