I did a quick google-search for you. This are the regular flea-markets I've found:
Flohmarkt-Sandhofen
68307 Mannheim, Amselstraße 10
Every Wednesday, 8:00 am
Kaufland (am Herzogenriedpark)
Mannheim-Neckarstadt
Every Thursday and Saturday, 8:00 am
ABB Tor 8
Mannheim-Käfertal
Every Saturday, 8:00 am
Flohmarkt REAL-Markt in Brühl
100 m in the south of Mannheim (Rheinau-Süd)
Every Wednesday and Saturday, 7:00 am
Don't know if you can get video-games at any of them. Hope that helps.