This Halacha is an excerpt from our Sefer
Buy me here or on Amazon.com
Buying Chametz from a store after Pesach:
When buying Chametz from a Jewish owned store one must verify that they have performed Mechiras Chametz before Pesach. The above requirement however only applies when buying Chametz that was manufactured before Pesach, however Chametz that was manufactured after Pesach may be purchased from any store. [This can be verified through looking for the manufacture date that is on the product.]
Jewish store that did not sell their Chametz: If one does not know when the Chametz was manufactured, and it is possible that it was manufactured before Pesach, then if the store did not perform Mechiras Chametz, it is disputed as to whether one may eat this questionable Chametz, as explained in D. Practically one is to be stringent not to eat the Chametz, unless it’s a case of great loss. One may however benefit [i.e. sell] the Chametz according to all opinions.
A gentile owned store: It is permitted to purchase any Chametz from a gentile owned store, anytime after Pesach.
All Chametz that was owned by a gentile over Pesach is permitted to be eaten and benefited from after Pesach. Thus, anyone who did Mechiras Chametz before Pesach may eat and benefit without restriction from any Chametz that belonged to them before Pesach. The Chametz of a Jew which was owned on Pesach is forbidden in benefit for all Jews. Thus, when buying Chametz from a Jewish owned store, one must verify that they have performed Mechiras Chametz before Pesach. The above requirement, however, only applies when buying Chametz that was manufactured before Pesach. If one does not know when the Chametz was manufactured, then one is not to purchase this Chametz in this store.
Stringency of some not to eat Chametz after Pesach unless it was manufactured after Pesach:
Some are stringent after Pesach not to purchase the Chametz of any store, even if it sold its Chametz, unless the Chametz was fully manufactured [ground to flour] after Pesach. Such products retain the symbol “Nitchan Vinefah Leachar Hapesech/Ground and baked after Pesach.” This is done due to several reasons: 1) They do not want to initially rely on the sale. 2) Perhaps the store purchased Chametz after Pesach from a Jewish retailer who did not sell his Chametz. Some communities are accustomed to be stringent in this matter until Shavuos.
May one buy non-Kosher for Pesach foods from a Jewish store that does not have a sign of Mechiras Chametz?
All foods that do not list any Chametz in their ingredients, and are not suspected to contain Chametz, may be purchased from any store even if the food is not labeled Kosher for Pesach and was owned by the Jewish owned store from before Pesach. If, however, one suspects the food may contain Chametz, then it requires further analysis as to whether it may be eaten, even if Chametz is not listed in the label ingredients.
May one eat after Pesach Chametz that is offered to him by a non-observant Jew?
Ø Example: After Pesach, one was offered a beer by a non-religious friend or relative. May one drink it?
If one knows for certain that the Jew purchased the Chametz before Pesach, then it is forbidden to be eaten. If one is unsure as to when it was purchased, then this matter is subject to the dispute mentioned in D, and practically, one is to be stringent as stated above.
Until when in the year is one required to verify that a store has sold their Chametz before Pesach, or verify the manufacturing date?
Ø Example: May one purchase a Chametz product in Elul from a store who does not have a Mechiras Chametz certificate?
This matter requires further analysis.
Purchasing duty free alcohol after Pesach:
One is to be careful to check for a Mechiras Chametz sign upon purchasing Chametz alcohol’s in Ben Gurion airport. An additional issue that Rabbanim have brought up in recent times is regarding liability. Thus, one should not purchase duty free alcohol in a Jewish owned store prior to Pesach and leave it in the store to pick up after Pesach by one’s return trip, if the store, or airport, retains liability over the Chametz alcohol if damages occur. See Chapter 2 Halacha 5C!
 If they have a certification attesting to the sale, then one may purchase all Chametz in the store, and is not required to suspect that perhaps the store bought the Chametz after Pesach from another Jew who owned it over Pesach, the same way we do not suspect that perhaps the Chametz found in a majority gentile area was owned by a Jew. However, some are stringent in this matter.
 Admur 448:30; This dispute applies even if one knows for certain that the Chametz was manufactured before Pesach, as perhaps the Jew bought it from the company after Pesach.
 Admur ibid
 One is not required to suspect that perhaps the store bought the Chametz after Pesach from another Jew who owned it over Pesach, the same way we do not suspect that perhaps the Chametz found in a majority gentile area was owned by a Jew.
 See Chapter 5 Halacha 1!
 A note on food labels: According to the FDA regulations it is possible to include Chametz as an ingredient without the consumer knowing of this. This can be done either due to the ability to list certain ingredients as a general term, such as “natural flavors”, or if the Chametz is a trace ingredient, which means a sub ingredient of the food that is “present at an incidental level and has no functional or technical effect in the finished product”. Thus, it is possible for foods to contain actual Chametz and not be listed. The ratio of this Chametz is not regulated by the FDA and hence there may even be more than 1.66% of Chametz [not nullified in 60%] within the product. Furthermore, regarding Chametz we rule that if it was purposely placed in the food as a normal ingredient it is never nullified even in a 1000x, even if done before Pesach. [442:6] Hence all standard Chametz ingredients placed in a food would make the food forbidden to be eaten after Pesach if owned by a Jew, irrelevant of the ratio of the Chametz. [See 442:6. Now although there it says that one can redeem the Chametz value of the food, nevertheless in 442:3 it is explained that this only helps to allow benefiting from the food [such as selling it] however the food itself remains forbidden to be eaten. Vetzaruch Iyun in a case that there is 60x the Chametz in the food, if redeeming the Chametz permits the food to even be eaten. See 442:6] Thus in conclusion, it does not suffice to simply rely on food labels and one must also not have suspicion that it contains Chametz.
 As there is no reason to suspect that it contains Chametz.
 As even by definite Chametz that was questionably owned before Pesach there are opinions who rule one may eat it, and hence perhaps if there is doubt as to if there is even Chametz in the ingredient one can be lenient, and so writes Nitei Gavriel 60:11 based on Admur 467:8. However Tzaruch Iyun as in 467:8 Admur permits the Chametz after Pesach only due to a Sfek Sfeka, and in this case there is only one Safek, whether it contains Chametz or not, and hence perhaps in this case one would need to be stringent, just as one is required to be stringent in the dispute of definite Chametz that was questionably owned over Pesach. [Whether or not there is 60x versus the Chametz is not considered another Safek, as when Chametz is placed as a common ingredient of that food it is never nullified even in 100x, even if done before Pesach. See 442:6]
Redeeming the possible Chametz: Redeeming the value of the possible Chametz does not help to allow it to be eaten, as this matter only suffices to permit benefit such as sales and not the benefit of eating. [See 442:3] Vetzaruch Iyun in a case that there is 60x the Chametz in the food, if redeeming the Chametz permits the food to even be eaten. See 442:6
 Vetzaruch Iyun if one can rely on the statement of the Jew as to when he purchased the Chametz.