What is a "Flohmarkt" - The translation I found was junk market (which, honestly, sounds a bit negative) or flea market. In the last 10 or 20 years these markets have been increasing everywhere. They usually are held in large parking lots on Saturdays and Sundays. Several of these flea markets will be held even 3 times a week.
There are special themes where people sell CDs, records, antique furniture or childrens' clothing. Others just offer anything you can possibly imagine. Even if you decide just to have a look and not to buy anything it rarely happens that you won't come home with some object or other. You can do great bargains you never expected, especially things like china, glasses, tools, games, books etc. I once got a great dog sofa for a ridiculous price. I didn't search for it - just found it.
Stuttgart offers a large Flohmarkt on the Karlsplatz every Saturday from 8.00 am to 4 pm. http://www.flohmarkt-karlsplatz.de/
For most flea markets in the region of Baden-Württemberg just try this link: http://www.marktcom.de/veranstaltungen-im-bundesland/Baden-W%C3%BCrttemberg or have a look at the local papers.