Good on you for giving it a try, it's something I've considered doing in the past. Sounds like you have everything covered. Are you going to offer framing? Though I guess this would only appeal to local buyers and would probably be more hassle than it's worth for you.
Out of interest, what's the going rate for a stall?
Have you got your banter ready? I reckon the Del boy look would work well.
Just a hunch, but I would imagine you will sell a lot of one particular print, there's always one that appeals the most. The trick is to work out which it is and have more copies of it available. You'll also need to push the prints that aren't selling so well toward the end of each day.
Let us know how you get on.