It sounds like a really cool, but terribly impractical idea. I would certainly go there a couple times, but I feel like it would become so gross, crowded and understocked that it would fail within a week.
Fat people would just bring a portable TV and camp out in the aisles for days, waiting for their favorite foods to be restocked.
The demand on the cooks would be way too much, since 90% of the things you buy in a grocery store have to be cooked/cleaned in a kitchen, or at least require some sort of preparation.
But yeah - I would go there once, just to see if it actually worked, but I don't think I would want to eat there, since buffets sorta gross me out.