There is a large market on Sundays, near the main international train station in Brussels (Gare du Midi / Zuidstation / South Station). Reputedly the largest market in the Brussels region, I have heard and read about it, but never got up early enough to visit. Finally, with the help of a few visitors, I managed to go, and regretted that I had not been there before.
I didn’t buy any items of clothing, though it was quite tempting since some of the things were rather dubiously cheap.
Being a glutton, I purchased fruits: mangoes, persimmons and oranges. They were cheap, sweet, and delicious.