There are people who collect militaria or items related to the military. Check out onlne groups or auction sites that specialize in World War II items. However, many people kept things from that important era so there are a lot of items out there. As baby boomers downsize, they often wish to divest themselves of sentimental things like the stuff their fathers saved from the War.
There is little value associated with ration books, German coins and stamps, military patches, uniform buttons, letters to and from home, or damaged goods.
Items that are valuable include the uniforms or insignia from elite forces. There is a market for German flags, armbands, knives, uniforms, helmets, and Japanese swords. However many auction sites will not sell Nazi items.
Check out sites that specialize in militaria. There you can find collectors' shows and other information relating to World War II. When you look for information, make sure to be specific in the description of each item. Words like "stuff" is way too general.