Originally, I started going to these as a buyer. Very short of money, would look at toy stalls, and then as my chlldren grew up and went on to Uni, looked for things for them to take with.. Crockery, cutlery, saucepans, mirrors, etc. etc.
Then I started doing them myself - by myself.. First one I went to = my car following a friends car (she was experienced), I was totally bemused by the 'trader' who descended on me as soon as I parked up - and started to take things from my car - I had packed it so carefully. From then on, I would just say to these people, very definitely - that I did not sell any items before I had totally set up.
Would take pasting table. clothes airer (to hang clothing items on), garden chair, packed lunch, sun hat, had a cross body bag - but notes, etc. kept safely in car. Most things priced so people did not have to keep asking me - but a notice saying 'If you like it - make me a reasonable offer'.
would ask neighbouring stalls to keep an eye on things when I made a quick loo run.
The one I mainly attended was very large, and attracted many people coming to look around. Back then only the stall holders were charged.
Once I had covered the cost of the place, I was in profit and always made enough to make it worthwhile for me. Would usually do two or three a year. In the past few years, I have been joining my daughter once a year at a late morning one (means we do not have to be there at the crack of dawn).
As this one always has lots of space between each car (at least it was easy for us to have our own tables side by side. But, she has now finished her de-cluttering - and even with her doing the driving and setting up/taking down the tables, I now find it too tiring - will not be doing any more.
It does seem as there are more and more actual traders as these boot sales these days, and fewer and fewer of genuine car boot sellers. Or perhaps I am just getting cynical.