Unless you have original paintings of extremely popular subjects or that are priced really low, originals will sell slowly. Most people can only afford prints, you will have a better chance of making sales if you have prints of your work available for sale. My biggest selling prints are 8"x10" matted to 11"x14". I also bring a few 12"x16" prints matted to 20"x24", but they sell very slowly.
Be sure your prints are archival. Just because they are printed on acid free paper does not mean they are archival. If the ink fades in a year or two, you are going to have some unhappy customers. Don't use your desktop inkjet printer to make fine art prints. Most of them use dye-based inks that fade almost immediately. You will need to purchase a printer that uses lightfast, permanent pigment inks with an estimated life of not less than 30 years, or find a printing service that has one.
Whether or not you number your prints is up to you. I think it makes a difference in my sales to have limited edition giclee prints with certificates of authenticity of my watercolor paintings. It does not seem to be important for photographic prints, so I don’t number those, but I sell them at a lower price.
Notecards are also good to have; sometimes you can make your booth fee just on sales of notecards alone. However, be very careful about how you package them. If given the choice between a $4 5"x7" notecard and a $25 8x10 matted print, most people will buy and frame the notecard rather than putting out the money for a matted print. Never sell notecards individually or in variety packs because they will compete with/lower the value of your prints. Sell them in packages of all the same image, or only sell singles/variety packs of things you do not sell prints of.
Next: How Are You Packaging Your Art?