Follow up to this question:
In almost all the Jewish-owned supermarkets that I have seen, the store owner completely seals the chametz aisles by placing a huge cardboard "wall" at both ends of the aisle and taping it so that no customers can enter it.
I somewhat understand that they don't want their customers to accidentally buy chametz by entering the aisle. However, I can see one problem with this.
If the non-Jewish chametz owner, Favolio, comes into the supermarket on Hol hamo'ed and says that he wants to take a case of linguini (he's making pasta for Pesach ;-), the owner would have to take down the wall for him, no? A bit bothersome, perhaps.
Also, if I understand correctly, the chametz may be visible, anyway, no? Doesn't it suffice to place a sign at the ends of the aisle saying, "Do not enter except for Favolio?"