Car boot sales are usually held in a field where anyone can come and sell unwanted goods. Most of the time these sellers pay a fee for their pitch and will set up a table.
Some people also sell cracked software on disks (tsk-tsk) and other things like tobacco etc.
The great thing about Car Boot sales is when the event's finished for the day and everyone packs up, some people throw some of their stuff in the skip (if one's available) and some really good stuff can be obtained for free if you're willing to look
Lol, it's like this post has turned into a language lesson
So I take it that the difference with a Flea-Market is that it's held in a large warehouse???