It isn't just the buying. They sell lots of Surplus stuff. But dont try to buy anything unless you are an insider or contractor. You will find lots of little rules and regulations that gerrymander all the best deals into the hands of insiders.
I just got a good dose of it when I tried to buy some used computers for the disabled at GSA Auctions. They have some proxy bidding rules that are daunting and a secrecy policy that hides the workings. It aint a real auction at all. Unlike a real Auction where the highest bid wins, there is no way to ascertain whether it is on the up and up or not, except to take their word for it.
It looks to me like contractors are pretty much in control of the government.