Yes there's a large Car Boot sale at Footzie's social club near Lower Sydenham train station on Sundays (I think it's Worsley Bridge Road?). Worth a visit, but if you're selling, then you need to queue early!
I once had a table at the Footzies car boot sale but didn't make much after paying the entrance fee. Nobody wanted to pay even a remotely reasonable price for anything and only popular things were CDs. If you fancy a day out it's quite fun and you get to meet some nice people who clearly do it regularly on a social basis but money-making it isn't. I think boot sales these days have been overtaken by Ebay and Freecycle so you might want to consider these or just give the stuff to a charity shop.