I am told by those that do these things that the cognicentii descend on newbies as they open the car boot and ferret around in your boxes in the car before you have set out your stall and generally pester you for prices when you haven't yet got it clear in your head. That assumes they are all perfectly honest but pushy and are not spiriting things away before you realise.
These are "traders" trying to get a deal they can make money on usually - or early birds getting the ---------

How you deal with this I have no idea but being early (they will be too) is not a bad ploy for one.