I've found on my non-hardcoe game that merchants only seemed to restock after a certain period of resting/waiting, not game time from exploring or fast travel. They'd also sometimes randomly restock after a load.
I turned off hardcoe and tested this, and it looks like you're right. This... is kind of a problem, because my character is hungry! =D I guess I should stop passing time by travelling and actually put in some time in the sack. I cannot afford the negative effects to health of just sitting in front of a store, hitting T and resting 3-4 days all the time! I was under the impression that travel time and rest time would be the same thing... apparently not!