Advantages: Great bargains to be had
Disadvantages: The unpredictable British weather
I love car boot sales. Some of the great stuff I have found there is amazing. I have found old books that I couldn't find online, toys from my childhood and lots of hobby stuff.
I love to haggle with the sellers. we have a bit of a laugh. Now as much as it is nice to get a bargain, it is not nice to offer a seller a stupid price for something. so if a chap is selling something for a fiver that is clearly worth around about that mark, don't offer him 50p. Most sellers who have thier stuff priced up have spent time doing this, so they aren't going to be time wasters. If they have spent the time doing this, they have probably spent the time finding out what their stuff is worth (i.e. see how much it sold for on the auction sites, then offer it for a half to to a third that price at boot sale). This is fair.
Summary: Car boot sales are a great way to spend a sunday morning when there is nowehere else to go