2 in the Swansea area I know of, one is on a Saturday in the car park at Mecca bingo Cwmdu, https://www.facebook.com/tyfforesthouse.swansea
And the other is the high street car park next to the train station. That is on a Sunday.
https://www.facebook.com/Highstreetcarbootsaleswansea
Never been to the mecca one, but judging by some of the comments I've read, doesn't sound too clever. The high street one is alright, pretty grim location but it does get busy there, if you decide to go there make sure you book, and also even if you book get there early otherwise if it's busy your space gets nabbed.
I did one a few year back at the high street, never forget this one particular woman, was selling curtains for 7 pound, she said i'll give you 5, ok then. She only then goes and puts them on her stall opoosite for a tenner. :lol: