The scenario of them keeping tabs on what we buy, is, honestly, preposterous. Lets consider this for a moment; the govt has their own watchdog groups to keep tab on their OWN departments. How well does that regulation work? Look at current events.
The logistics behind keeping tabs on large purchases would be literally insurmountable. Besides, its easily circumvented-just purchase a little more than you need each shopping venture, and soon enough you will have a reasonable stockpile.
Think of small eateries that buy their food in bulk as well-my stepmother owns her own cafe, and buys the groceries in bulk on her own CC. The food isnt kept at her house-its at the diner. So, the govt would simply waste time searching the house. Again, assuredly, that wouldnt be an isolated incident.
And, as others had said; if our society broke down to that point, I dont think aquiring civilian food stashes would be high on the list. In fact, I would assume that list no longer even existed-if it ever did at all.