There are great fruit and vegetable markets (Markt) all over Karlsruhe. Check this local website for specific information but remember that markets will close at 2pm and start packing up around 1pm. Be sure to visit:

Fun and sometimes fabulous, Karlsruhe has regular fleamarkets (Flohmarkt) on a weekly and monthly basis as well as hosting a large market 3 times each year. There’s a Stadtwiki link which lists current dates.

Be sure to look at other fleamarkets in Germany and in nearby Alsace as well.