Harry is an asshole

But creation of food is against the rules. It's one of the main three "Can't do that"s.

maybe it just restocks from a store and they just pay later

Even if he did buy everything. He bought it, it now belongs to him, he has no obligation to leave some for others (not like he even got to eat candy before).

Well, no obligation, but if he did actually buy it all he is an asshole.

If there was one brownie left, or you apply for a job there is only one- are you an asshole to take it because someone else can't have it now? So it's not the taking that is at issue. That leaves quantity."all" is "all" be it 1 or 100. However there's more to it. Need, and immediacy are factors (i.e.: taking food from someone starving because you are a little hungry is not kind. They NEED it, you WANT it.) but it's candy, so it's all "want." So the question becomes- was it wasted? If you take more than you use then others could have used it- but since it's unlikely the cart would sell out if Harry didn't buy it all, there would be unused candy in that scenario. He didn't buy it out then sell at a profit, or throw it out. So while asking her to come back after the other kids had a chance and buying what was left would have been more polite, those kids would take first pick of the "best" candy and then when he bought it would be leftovers. So in that scenario are the other kids jerks?

I get where you are going philosophically and mostly I agree but buying out the candy trolley on a train full of kids is still a dick move.