Fear. I think potential customers can smell fear. Any weakness makes your product less viable, and so it may not sell as well.
My advice? Put your game face on and get out there!!! Smile. Engage people in small talk. Find something to genuinely compliment someone on--their hair, dress, shoes. Make an effort to be affable, and don't frown at anyone. I know...these are just common sense, but you'd be surprised by the number of vendors who are condescending to customers. People often say things to me which make me feel irritated, but this is usually through no fault of their own. They can't be knowledgeable if YOU don't gently educate people about your product. Explanations given with a smile bring smiles. It's that simple.
Knowledge rids one of fear of the unknown.
Never be afraid to be you. I joke, tease gently, and am supportive when necessary with everyone I meet at shows--vendor or customer, artist or craftsperson. Makes no difference. We are ALL in this together, trying to survive and get what we need. If you, as a vendor or customer, can bring a little joy to someone's life--why not? What's to be afraid of?
Bottom line: Kindness costs nothing. Smiles cost nothing. Fear can be banished by both. If I can do these things for anyone, I will try.
And in doing the above things, I can assuage my own fears and become stronger. In Dune, the author says 'Fear is the Mind Killer'. I so believe this. It is the root of all wars, racism, hatred, and genocide.
And is inherent in the way we present ourselves to the world. Fear wreaks havoc with your inner peace, and will come forth in the way you present your work(which, as an artist, is a part of my soul) to others.
Replace fear with gentle pride, kindness to others, and goodwill. People will be drawn to you, and you to those who also share your ideals.
I know--easy to say, harder to do. But try. If we all tried to rid the world of useless fear, think of how beautiful the world would be! And how much more productive!