When one sells his chametz should he sell the chametz in certain areas, or just sell the entire apartment? Is there any reason to do one over the other?
One reason to sell chametz in certain areas, and not in the entire apartment, is to ensure that one retains an obligation of bedikat chametz.
If chametz in the entire apartment is sold, then according to many authorities (as cited by the Mishnah Berurah, concerning whether a sale on the 13th or the 14th exempts from bedikah) there will no longer be an obligation of bedikat chametz, because chametz will not be found, it will be the non-Jew’s chametz, and not the Jewish homeowner’s.
Note that in most sale documents, the shtar mentions the specific places, and adds “and anywhere else in the house,” which means that this benefit is not achieved. Some, for this reason, are careful to leave out one room from the sale.
Another reason is so that the sale should be more “authentic.” It makes more sense to sell chametz that is located in a specific location, that to sell the wafer that is located “somewhere in my house.”
However, selling chametz anywhere in the entire house is also effective.