There are many people that collect Military Items and have been doing so for well over a century. They collect from different eras, different countries, and different branches of the military as well.
Military collectibles for the most part can be expensive but well worth the money spent, because now you are really talking history. When a collector gets a piece of memorabilia he is getting a piece of history. If he obtains a uniform from WWII, he must wonder what the soldier was like, where did the war take him, was he injured or killed during the war.
Medals and awards are huge for collectors, and to find one in nice shape is even better. I have some medals of my Dads from WWII and his time in the United States Navy and knowing he was a part of the D Day invasion at Normandy France on June 6, 1944 makes these items even more special. I have pictures he took from the back of the ship, showing some ships in the convoy on the English Channel, in some pictures you can see the bombed out buildings by the shoreline of a river somewhere in Europe.
As with many collectors we keep them as well as his Medals from the Korean War in a shadow box for display, keeping them clean, and nice. One medal that cannot be sold ever is the Congressional Medal of Honor, generally this award is presented by the President of the United States, and is award to a member of the Armed Forces who distinguishes himself conspicuously at the risk of his life above and beyond the call of duty while engaged in an action against any enemy of The United States. The deed performed must clearly distinguish the individual above his comrades and must have involved risk of life.
Many of the German Badges and Awards are highly collectible.
Some places to get information on the German badges is Forums for collectors, they are very serious and very knowledgeable about the items and can be of great help to anyone looking to see if their item is the real deal.
Another group of Military collectibles people like are Field Gear, such as Radios, First Aid Kits, and other equipment. Collectors also like documents from the war, you can find actual papers from the war, orders, maps, secret plans as well. Some are tough to find in nice shape but they are out there. Pictures are also a nice collectible, Time Magazine was big for publishing War time photos, mostly what you will find is black and white photos. And many people will find, usually in old scrapbooks, pictures that the actual soldiers took while in the military during the war.
War time books are also a great collectible, and they come in many forms. You can purchase books about the wars, that explain the history and strategies and leaders involved in the wars. Not just WWII, but WWI, the Civil War, the Revolutionary War, Vietnam and more. Or you can find books written by actual leaders like Mein Kampf by Adolf Hitler, and Erwin Rommel the Desert Fox who wrote Infanterie greift an, which means Infantry Attacks in English, and is the book that General George S Patton read to get a better understanding of his biggest adversary.
People also collect postcards about the military, there are ship models and model tanks and more. Games about the wars are also popular. So as you can see Military collectibles are quite extensive as far as what is collectible, basically everything. And it seems like WWII is the big one for collecting, because it basically enveloped the whole world and for obvious reasons had worldwide attention. So collectors of WWII memorabilia can be found in all corners of the world. Another popular time for the militaria collectors at least in the United States is the Civil War, the Civil War has much intrigue. Imagine a country split and brother fighting against brother.
You can find many books with great stories and pictures about the Civil War. There are many weapons from the war also that collectors love to restore to a nice condition. In the United States the items are very popular and there are reinactments of battles all the time, but you have to watch out for reproductions on this stuff. Then again you have to worry about reproductions on any collectible, the reproductions are nice and clean, but it's just not the same, and their value is nothing compared to the real thing.
My best advice to anyone interested in getting involved in Military collectibles, is to do a good amount of research before spending money on items. Make sure you go to the library and read books on these collectibles, how to spot authentic items, what they are worth. Check price guides at the local bookstore to find out how much you should pay for an item also. Find out in forums what to look out for to make sure an item is authentic and not a reproduction. And once you find that item, if it's not already in nice shape, fix it up or clean it up, and display it with pride, but protect it, you're not just protecting a collectible, you are protecting history.