If you have a portable power pack just in case your phone and/or card reader gets low on juice, that can be really handy.
Make sure you have a secure place to keep your change. A proper metal cash box is really handy because it keeps the money safe and has little trays for your different coins, which speeds things up a lot.
Little notebook and a pen to record how many of what you sold and to tally up numbers as you go or make notes about trades (sometimes people will trade you an item for one of the same value from their table.
Paper bags to put purchases in are handy!
Fruit juice and energy bars can be an absolute lifesaver when you've been on your feet all day.
Oh and something to keep other people's business cards in.