The military gets rid of their old, used up stuff, through several channels. What normally happens is that troops turn in used gear to a civilian run facility (I believe we used to call it the DRMO, but I may be wrong). If it is deemed unserviceable, then it is put aside. From what I understand, civvies get first crack at buying them. Or simply take them home. They will then resell it to surplus stores. Thats one way it makes it into the stores.
The other way is simply troops selling their gear back to them, or to other people, who eventually sell it to a surplus store. I have sold a few items, having them written off as "losses", as I could make more money selling it than it cost me to replace it through the military.
One other way is that the US has large inventories of gear that will never be issued. Sleeping bags, uniforms, mess kits, old web gear-it takes up space. They sell it to a contractor, who in turn sells it off to whoever.
I agree with Hikermor-beware of what you but. Some of this stuff is 50+ years old. Old sleeping bags may be in poor shape; old metal frame rucksacks may have broken frames (I snapped 4 in my career alone), and the older cotton gear may be rotten. Its best to have the item in hand, and physically inspect it, before purchasing something. Not to say that there arent great deals-I recently bought a wool blanket, issue, brand new, for $20 I think. Never used. Deals ARE out there-you just gotta go with a good dealer.