You load your stuff for sale into the trunk of your car and set off for your chosen car boot sale early in the morning, Saturday or Sunday, pay your fee to pitch, set up in rows with a fair crowd of others, and sell your gear to whomsoever rolls by. Antique dealers, general public, trolls.. It can be great fun and profitable. One time I made £250, another time £32.. That wasn't so great but at least the stuff was gone.
So kind of like a flea market I spose. Open to anyone, and some folks take it Very seriously and make a regular secondary income, like a market stall holder.