The only thing that allows this is lag, so it won't work in single player and works better with higher pings or a server with a lot of people. When you enter a server at first you can do this with any specialty except sniper and I think demoman (because the amounts between the weapons are too much). This description is with heavy gunner. Buy the first weapon and then VERY quickly go over and buy heavy armor (you don't actually have enough money, but lag will cause a slight pause after you buy the gun giving you enough time to get the armor). If it doesn't work at first sell the gun and try again. When you get it, sell the gun and click your armor back down to medium, and BOOM! You have more money than you started out with! Now repeat, except get the second gun. After that you should have enough money to get the third one, and then the fourth. Now when you get the fourth gun and the heavy armor, you should have around $7100. If you sell the gun and armor, you'd get about $7900. I always stop at that amount and get my gun or whatever for the sake of time, but you can actually go up to $13000 or something if you do the shotguns and stuff. You can do this as long as you have $300, which is enough to get the weakest pistol.