Well it depends, if they are rare-ish then I suppose so, but I wouldn't really pay anymore than 50p for a 7" and �1 - �2 for a 12". A lot depends on how much you want to get rid of them.
yeah they gotta be �1 or people wont bother looking through. its the whole point of going to a car boot sale to buy records. its cos they are cheap. you can easily go to an oxfam music for rare /old stuff so why pay more at a boot sale.