Car boot sales are the British version of the flea market and the yard sale, tens or hundreds of people gathered in a muddy field to sell their cast-offs from trestle tables set up at the back of their car. Like everything else in Britain, there are subtle class distinctions and snobberies and all kinds of silent etiquettes that one instinctively observes. But at a basic level they have an admirable aim; to earn the seller a bit of pocket money selling their 'trash' for someone else to 'treasure'. It's recycling from before when recycling became a buzz word.
Generally speaking, you do have to rummage around a little to find the treasure. An increasing number use the car boot sale as a last ditch attempt to get a little cash for something before dumping it. Currently the secondhand route is approximately as follows:
Prized Possession --> Private sale --> Small Ad/Gumtree --> Ebay --> Boot Sale --> Charity Shop --> Bin
People miss out stages depending on the quality of the goods and their general laziness. Sadly, a lot of people bypass everything between 'prized possession' and 'bin'. In fact, a staggering number don't appear even to value their possessions when they plan to hang onto them. Then there are a large number who don't have the time, or don't need the money, so tend to dispose of everything useable to charity shops. I like these people because I like buying their cast offs! But a lot of people will try and sell their stuff by any means possible before resorting to a car boot. Prices are low and it's one of the few places where us reserved Brits will haggle.
Conventional wisdom has it that you have to get there early to get the good stuff. It's true that the 'dealers', that is, the professionals and resellers, will be there early and will search aggressively for what they are after. This morning I witnessed a dealer literally snatch a large box out of a lady's hands to search through it before anyone else got to it. (I felt a little surge of joy when she tartly told him she wouldn't sell a thing to him at any price after that, and all of the nearby stallholders applauded). If you don't get there early you'll never get any of the urban legend-style bargains. But to be honest there was never anyone selling still-boxed Star Wars toys for 20p each anyway. If you're happy to scrum, or you're after something specific like Lego or Barbie dolls, then yes, you need to be there at the crack of dawn. If you just want a poke around through other people's junk and you prefer it when people behave in a civilized manner, then early-ish, an hour or so after opening, is fine. You can take your time wandering round and have fun even if you don't make your millions from what you find.
If you're after the biggest bargains, then you should ignore the 'rules' altogether, and turn up late. An hour from closing time is perfect. Although there are professional booters who'll pack up and bring their unsold items back next week, the vast majority are householders who've had a bit of a spring clean and would really rather not cart all their rubbish back home again. Many will accept any offer to get rid of stuff. I even know professional booters who get all of their stock this way, by offering £5 or £10 the lot just as the sale closes. And the seller tends to be really grateful for the offer. Of course, there is a trade off; thousands of people have already picked over and rejected this stuff. But if you're a visionary who enjoys the challenge of transforming the unloveable, it's definitely a strategy to consider.
I personally use both strategies, depending on whether I am looking for something specific and whether I can be bothered to get out of bed early enough. This morning I was looking for Scrabble, but it was such a lovely, peaceful morning that simply being there was enough.