Thats the thing with small busineses like this it can be difficult to break even, you might not make enough to pay tax but you will still have to fill out a tax return.
A lot of people starting businesses like this based on a hobby or an activitiy they enjoy don't factor in their time. So you could be slaving away all night baking and never get the money for that time. That might be fine with you if you really enjoy it, but the more you sel the longer you spend doing prep but still never cover the cost of the hours you work.
You really have to do realistic costings for a 12 month period. It is guess work at first trying to work out how much you will sell or at what price. You have to work out how much you will need to sell.
So after considering pitch rent, insurrance, ingredients, packaging, labeling, time, travel, electricity etc etc for week then decide how much profit margin on each item you will aim for. Some costs will change according tot he amount of stock like ingredients, some will stay the same like rent.
You could do a few different costings varying the prices and amount sold to find a projection that seems to give enough returns, then consider if this is relistic.
Oh and check out if any competition, if you have simliar well established stalls it can be difficult to arract the customers which can be very loyal.
Could you acheve the same profit margins with something you buy in, less legislation and prep for the same profits.
I strted off with sweets that I packeged my self in hand made cones, progressed to packing in premade bags, then went for pick and mix and prepacked sweets, then discovered some good suppliers for toys which had a simliar profit, but no prep, no environmental health to worry about.