Things I learned from my first flea-market experience
Bring a chair (my lovely friend's hubby gave me one to use and I was very glad of it).
Always have someone around (next door stallholders if nothing else) to look after your stall while you nip off to the toilet.
Bring food and drink. No point in blowing your entire profit by eating from the inevitable food stalls and chip vans.
Bring some kind of transparent plastic sheets to cover stuff if it starts raining (or a pavillion type thing if you have one). The cheapie plastic sheets you can get for covering furniture while painting would be good for this - I saw lots of people with them. I did think to bring rubbish bags but that mean my stuff was covered when it was raining and no-one could see anything, even though people were still walking around under umbrellas.
Almost as soon as I arrived to start setting up I was approached by a man asking if I had any jewellery for sale (I didn't). Then a few minutes later another one. And then another after that. I assume they're traders of some sort who do the rounds first thing hoping to snap up any bargains. Must ask someone with more experience of flea markets whether it's a good idea to deal with these guys or better to keep the stuff and sell to the general public.
Having a float of some change is a really good idea. I probably overdid it with 40 of notes and 45 of change but at least 20 or so will make life much easier.
Decide whether your priority is to make money or to get rid of stuff. If it's the latter, just price to sell. Yes, everyone says you have to haggle but not everyone likes to and if the point is to get rid of stuff...I admit it, I'm a bit like Brian in this respect.