It varies from seller to seller. Some are individuals selling off the contents of their sheds, some are traders buying & selling on a regular basis. Just play it by ear when you get there. I'm sure you don't need me to tell you, but obviously don't flash large wads of cash about, don't hand over a deposit without a receipt & a damn good idea of where the bloke with the vanful of bike bits lives & don't take anyone's word for the condition of anything -check any prospective purchase over thoroughly & go on just what you can see, (a rusty old bike is still a rusty old bike, regardless of the seller's entertaining sales pitch).
But have fun! Don't go determined to bring a bike home with you at any cost -there's always other jumbles & some Kempton ones are much bigger than others. Just enjoy!