Military pins and military medals are a very popular collectibles.  Although I've seen a few of these being sold at estate sales, I don't believe I've ever seen any truly high quality collectible military pins being sold at yard or garage sales. Vintage military items are wonderful to collect, but they are often held on to by family members, and so, can be hard to find.

If you are looking for items to sell, military lapel pins and military hat pins are both popular among collectors.  Of course, among these, there are certainly some that are more valuable than others and there are also other considerations to take into account.  Recently, I saw a couple of Honorable Discharge medals being sold with their original boxes that went for over $400 each.

If you are buying, well, you certainly know what you are looking for online.  Be sure to ask for additional close up photos if possible before investing too much.  Also always ask to see all sides of the item.  Never assume that because something looks great from one view, that it will be in wonderful condition all around.  Most quality sellers will be more than happy to show you every detail and answer questions about age, condition, history, and the like.