If you have a lot of money, and have unlocked a particular gun, you can buy that gun and sell it for a very reduced price. However, when you sell the gun, whatever ammunition inside it when being sold is be transferred to your inventory, allowing you to "buy" ammo, albeit at a very high price Note: Buy the lowest cost type of gun because the ammunition is universal. For example, it does not matter which pistol to buy, as long as you are trying to get pistol ammunition.
