Eli from Monsey, NY wrote:

Dear Rabbi,

In my neighborhood there's a big supermarket which is part 
of a chain that is supposedly owned by a Jewish person.  
This store is open on Passover and sells chametz.  Assuming 
that the owner really is Jewish, that means that I can't 
buy chametz there after Pesach.  Right?

Dear Eli,

Chametz owned by a Jew during Pesach is forbidden after 
Pesach, even if the person owned the chametz "accidentally" 
-- e.g., he forgot he had it or he didn't realize the 
severity of the prohibition of owning chametz on Pesach.

So you're correct:  If the owner of the supermarket is 
Jewish, then it is forbidden to buy chametz there after 
Pesach.  Rather, you have to wait until enough time has 
elapsed for him to be able to sell his entire stock of 
chametz and purchase new chametz.

Certain supermarkets of Jewish ownership in fact follow the 
custom to sell their chametz to Gentiles for the week of 
Pesach.  Some Halachic authorities question the validity of 
this sale, however, since a non-observant store-owner 
probably views the sale as a mere formality and not as a 
binding contract.  Especially if, after selling his chametz 
to the Gentile, he turns around and sells the chametz to 
his customers during Pesach.  

Rabbi Moshe Feinstein rules that the sale is clearly valid.  
When the owner goes ahead and sells some of the chametz to 
his customers, he is simply re-acquiring that chametz 
(although he may very well be considered stealing).  The 
leftover chametz, however, remains the property of the 
Gentile and can therefore be eaten after Pesach.  This is 
because the owner *wants* the sale to the Gentile to take 
effect wherever it can, in order to "save" himself from the 
transgression of owning chametz.  Even though we see that 
he is *prepared* to go against the laws of Pesach, he does 
so out of a motivation for personal profit and not because 
he *wants* to violate Halacha.  Wherever he is able to keep 
the Halacha without losing out, he wants to keep the 
Halacha.  So, concerning the remaining chametz, he wants 
the sale to the Gentile to be valid..

However, Rabbi Feinstein points out that it is nevertheless 
forbidden to buy chametz from such a store after Pesach.  
Why?  Because the sale applies only to chametz owned by the 
store at the time of the sale.  Any chametz the store 
purchases later -- i.e., during the week of Pesach -- is 
not included in the sale, and therefore forbidden during 
and after Pesach.  Unless the store receives Rabbinical 
certification that the chametz was sold before Pesach and 
that no "new" chametz was brought in during Pesach, it 
would be forbidden to buy chametz there after Pesach.

In Israel, the circumstances are somewhat different, since 
the stores generally do not replenish their shelves with 
fresh chametz during the Holiday.  Therefore the only 
relevant consideration is the validity of the pre-Pesach 
sale of the chametz to a Gentile.  Rabbi Yosef Shalom 
Eliashiv, shlita, insists that special clauses be added to 
the standard contract for the sale of chametz in order to 
emphasize the seriousness of the sale and ensure that the 
sale is "for real."

The custom of the Vilna Gaon was not to buy any chametz 
that was sold for Pesach.  This was based on a concern that 
the sale of the chametz was not 100% valid.  Although some 
people today follow this custom, it is a chumra (Halachic 
strictness) and not the standard practice.  Furthermore, 
some people only buy chametz made from wheat which was 
ground into flour after Pesach.  This is due to a concern 
that the flour ground before Pesach may have come into 
contact with water.

BTW, why was the baby chametz-cookie crying?  Because his 
mother was crummy and his father was a wafer so long.

Sources:
 
  Shulchan Aruch, Orach Chaim 438:3.
  Halichot Sadeh in the name of HaGaon Rabbi Yoseph Shalom 
Eliashev.
  Iggrot Moshe, Orach Chaim 1:149, 2: 91.
  Siddur HaGra.


