First of all, any writer attempting to sell books at a festival needs eye appeal. Since I was offering books about ghost stories, I decorated my table with a spooky tablecloth, skulls and other such props. These catch the attention of passersby. Also, I had posters with the covers of my books sitting next to my table. These are good conversion starters. A simple question then gets the ball rolling, if someone stops or hesitates: "Do you like ghost stories?" If someone answers "yes", then I offer my elevator pitch. Every author worth his/her salt has this ready. It's simply a quick pitch to sell your book.
I also have business cards available for anyone to pick up. For whatever reason, a person may not buy directly from me, but might order or visit my website later. For this reason, a writer always needs to stay pleasant. Not everyone who stops will buy - right then. You may pique their interest and they'll order at another time. Be courteous and conversational. You just may hook a reader for life, who will look forward to your future works.
Of course, you need to keep other factors in mind when working outdoors. Wear appropriate clothing, have a canopy for shade or cover from rain, promote the event on social media and traditional media and maybe offer freebies like candy or inexpensive trinkets for children. The bottom line is to sell, but not be pushy. People are attending to have a good time. You should too. Enjoy the moment and have fun. For more tips on selling at outdoor festivals, visit the blog of freelance writer Amber Lanier Nagle.