Did 3 shows last summer outdoors and 2 months (Nov & Dec) of Saturdays last year and did not get one back check....Like Marvin said you can't prejudge a customer by their looks.... I had one guy come to my booth looking like he couldn't afford a free glass of water... He looked at my stuff, asked a load of questions and I treated him like any other customer... the following day he came by in a suit I couldn't afford in my best single days... and bought over $300 of stuff... all cash.... You never know....
Treat them all kind and answer all questions even, and if you need to deal with another customer who came in just excuse yourself for a minute and then get back to them...
Remember that you are not only selling your art, but yourself... If the customer takes a dislike to way they are being treated no matter how beautiful your stuff is they are going to walk away....
Tom