Problem with a lot of car boot sales is that you get people who go early and pay extra to get in before it opens.
These people go and find all the best stuff and then take it home to sell on ebay.
You also get the dealer type that lordspunky mentioned who will go round all the stalls as they are being setup and only set up their stall when the doors open.
They either buy things to add to their collection or add to their stall.
So it is very hard to find a bargain when it comes to computers.
Saying that though I managed to get a Commodore 128 and a Grundy NewBrain at a carboot for £8. Both working