eBay's a great place to find and start collecting militaria, but you have to be aware of what you are looking for and how to distinguish what is real and what's not. Most people concentrate on a small specialty niche for their collecting, i.e. Special Forces, Airborne, WW II, Vietnam and so forth. I think that researching your subject should be the first thing you do. There are many great books on just about any subject you can imagine and a very shrewd choice for your first few puchases should be these essential references. I have several M-1 helmet books, WW II US Army uniforms and patches books, and these will help tell you what era an item is from and markings and other features that help identify a real item from a fake.
    Remember that the rarer or more "trendy" niche items will either rarely be offered for sale or will be really expensive. Original US Army Airborne jump suits are pretty rare these days and when they come up are usually up in the thousands by auction close. German items of almost any kind are sought after and are more expensive generally than US or British items. You can be a savvy buyer by watching items and seeing what they average in cost, trying alternative auction listing names (para, paratrooper, paratroop, parachute, chute for Airborne items), and I advise using an auction sniping service. I know some people don't think sniping is "fair" (whatever that means), but it is a really good way of winning auctions that you really want to win without prematurely driving up the price. After all, we all want a deal.
    There are some disreputable sellers who try to pass off reproduction items or items from a wrong time period as a more valuable original. If you do your research you can usually spot these from the item listing's pictures. If you don't ask questions about an item then the burden is on you. There are deals to be had, but if you really want a particular item and it is rare or otherwise desirable, be prepared to pay through the nose. Collectors can be really rabid about their passion.
    A few last minute, general hints. World War II items are getting more and more scarce and most sizes in uniforms are around a 36 - 38 regular, so don't expect to find a wearable M-43 field jacket right out of the gate. Vietnam items are more numerous but are collectable due to the age of the collectors and the personal nature of the conflict to those collectors. Vietnam camo items can command stiff prices and are more rare than the OD uniform items. The latest media trends can affect desirability and price. "Band of Brothers" and WW II US Airborne gear. "Saving Private Ryan" and WW II US Ranger gear, well you get the picture.
    The best part of all this is to have a little fun with your collecting and learning a bit more about the history of the items and the men and women who used them .