The problem is that balance is very subjective and you can easily throw off the balance by playing differently than the developers had in mind.
The only way to maintain the balance would be to either restrict the player so much that he can't play differently than intended or to constantly monitor all of the player's progress and adjust the environment accordingly. For example, the trader could only sell what you can craft anyway. However, this would mean that the game would have to check your progress before showing you the trader's inventory. Or you find too many armor magazines, then the game would have to adjust the loot so that you only find a certain amount of armor magazines per day.