Not done boot sales as such but regularly sell soap at local fairs.
Definitely take lots of change, take a packed lunch, be ready to 'read' people - ie. some want to banter, others avoid eye contact altogether and they just poke through your stuff and shuffle off
If you have a pop up gazebo take it! take somethign to anchor it down to the ground/car too - saves getting all your stuff soaking wet should it rain.
I get cross with the other stall holders hanging about as I set up, I find it a bit stressful as I need space and time to make the display look nice so I'd agree wtih the others about saying you aren't ready yet but look forward to seeing them a bit later.
take a calculator if mental maths isn't your strong point (like mine!) maybe take some carrier bags, good way to recycle them and useful for your customers.
I get my kids (if they are there) to hang about the stall periodically as for some reason it draws the crowd over if it looks popular, soon as they see customers the girls know to mooch away so they can see my stock clearly
Keep smiling, enjoy yourself and anything you take over pitch fee is a brilliant bonus