I've not been to a car boot sale for years now but actually used to work on one with a family friend back in the early 2000s selling mainly stationery but all sorts of odds and ends (one of the most popular 'lines' he would buy factory reject socks in bulk and sell them at 50p a pair - these were actually good quality socks worth £3 - £4 retail a pair probably just with minor flaws you wouldn't even notice...and yet you would still have punters trying their luck with '3 pairs for £1 mate?!'

)
They've always had 'dodgy goods' in their DNA just the type of which has changed over time with the pirate CD/DVD period perhaps being the most prolific, used to chat to a few of the 'traders' who ran such enterprises and they basically just bunged the operator a few quid every week so they got a tip off if Trading Standards were on the way!
Even the stalls selling something as simple as chocolate these days is potentially just nicked from the local supermarket!