You can choose what, when, how much, and from whom, but you are still are still forced to do so. Choosing which person puts me at gunpoint doesn't make it voluntary
You can also feed yourself by growing food or hunting. Neither of those are banned, just more inconvenient and you probably have some other skills to sell and buy food instead
If I choose not to I die. I can buy the food from someone who already has it, I can buy the right to make my own food, or I can choose to starve to death