First you have the usual Galeria Kaufhof and Karstadt that carry quite a lot of boardgames, esp Queen Games, at excellent prices. Note that different sites of the same store charge different price for the same game, so you might want to check a few before purchasing.
Otherwise you have Mage Store at Graf-Adolf-Straße 41 not far from the airport that have excellent prices and selection.
So my wife hase moved to Dusseldorf and I will be following in May. Where are the best places to buy games?? I know I will be able to take great advantage from amazon.de but I prefer seeing the games and getting a test play if possible.
Also any recommendations for boardgame nights or groups are also very welcome.