1. Is selling Chametz a Biblical/Rabbinical requirement?
One who has cleaned his house from any known Chametz: One who has cleaned his house from any known Chametz is not obligated to do Mechiras Chametz. Nevertheless the custom today is to be extra meticulous and do Mechiras Chametz just in case some Chametz has remained in his possession without his knowledge.
2. Where is one to store his Chametz that was sold?
All Chametz which has been sold to a gentile must be kept
May one sell actual Chametz and leave it at home during Pesach?
Yes, and so is the Chabad custom. However there are those which are particular against doing so.
May one enter into a Chametz room or closet which was sold if he needs to retrieve something from there?
Yes. However one may not use the area regularly as if it were not sold, and rather the allowance is only to be allowed to enter on occasion to retrieve an item which one needs.
May one leave Chametz in his fridge or freezer and close it off?
No. This applies even if he wraps the Chametz well and writes sold or Chametz on it. However there are Rabbanim that are lenient in this matter.
May one leave sold Chametz in his kitchen cabinets?
May one leave Chametz in the cabinets under his counter?
Yes. However some write against doing so as if one does so he will no longer be allowed to use the counter for his needs being that it is rented to the gentile.
May one sell his Chametz through a Rav in a different time zone?
One is to sell his Chametz through a Rav that is in the same time zone that he is in. If this is not possible one may sell it through a Rav found in a later time zone. One may not sell it through a Rav that is found in an earlier time zone.
If a person has traveled to a different time zone for Pesach, where should he sell his Chametz?
Example 1: If an Israeli traveled to America for Pesach does he sell his Chametz in Israel or in America?
Example 2: If an American traveled to Israel for Pesach does he sell his Chametz in Israel or in America?
One who owns Chametz in the area which he traveled from is to always do the sale in whichever area is the earlier time zone [east versus west] whether he traveled to an earlier time zone or later time zone. Thus in both examples above, if one has Chametz in his home area he is to sell his Chametz through a Rav in Israel. [Although when doing so in the first example one should specify that he is only selling him the Chametz in that time zone, otherwise he will not be able to eat Chametz even before the 5th hour, as it was already sold.]
A further example-Flying from east coast to west in America: Thus when even flying for Pesach from the east coast to the west coast in America, or from the east coast to the middle of America, one needs to make sure to sell his Chametz in accordance to the time that Chametz becomes prohibited in the east coast. Similarly when flying to Hawaii from America for Pesach one must be careful to sell his Chametz by the Rav in area which he currently lives.
Flying from west to east in America: He must sell the Chametz in accordance to the time of the east coast.
Regarding buying back the Chametz after Pesach there is no problem for the Chametz to be bought back in a place that Pesach ends earlier then the time it ends in the place that one lives, as one has no intention to buy back the Chametz until after Pesach ends for him. However some Poskim rule that one is to sell the Chametz in the earlier time zone and tell the Rav to not buy it back on his behalf from the gentile until Pesach ends in his later time zone.
May one begin to eat Chametz after Pesach prior to the conclusion of the sale?
 Seder Mechiras Chametz of Admur, mentioned also in 436/18 regarding an oatzar of chitim, and in 445/2 regarding making hefker, however there he says that he only transgresses Rabinically.
 This applies even though one does Bittul to his Chametz, as the Chametz which he plans to sell is not included in the Bittul. [ibid]
Other Opinions: The Chasam Sofer, as well as other Poskim rule that all Chametz included in the bitul, thus according to them the sale of Chametz is only Dirabanan. See Shutim below for the nafka mina in this regarding when the sale should be done
 Background from Admur in Seder Mechiras Chametz:
“One who thinks that the selling of Chametz is only a Rabinical requirement, being that he already did bitul before the sale takes place, is mistaken, as the bitul does not include any Chametz that one plans to retrieve after Pesach. Thus if the sale is invalid one transgresses a Biblical command of Baal Yiraeh and Baal Yimatzeh.”
 Piskeiy Teshuvos 448/14; 436 footnote 20
 For this reason in some contracts they write that the Jew has retained the right to enter the rented areas if he needs to.
 As the Chametz must be behind a Kosher Mechitza of ten Tefach
 Piskeiy Teshuvos 448/11 footnote 50; Based on Mishneh Halachos 8/189 regarding bathroom cabinets; and so is the custom!
The reason: Although the cabinets are not ten Tefachim tall, as is required by a Mechitza, nevertheless it is valid being that we consider the wall that the cabinet is attached to, to be the Mechitza. [ibid]
 Rav M.S. Ashkenazi of Kfar Chabad
 Rav M.S. Ashkenazi of Kfar Chabad
 Meaning an area which is currently further ahead in time than one’s current area.
 As one’s Chametz must be sold prior to the 6th hour of his current time zone.
 Igros Moshe 4/94; Minchas Yitzchak 7/25; Piskeiy Teshuvos 443; leaning opinion of Rebbe in Shulchan Menachem 3/229 being that we follow the area of the Chametz
This matter is subject to the following dispute:
First Opinion: There are opinions [Oneg Yom Tov; Kaf Hachaim 443/17; opinion of Rebbe ibid] which rule that one goes after the area that the Chametz is in whether to be stringent or to be lenient. Thus if one travels from Israel to America he must sell it before the 6th hour of Erev Pesach enters in Israel.
Other Opinions: Others [Many Shutim. See Piskeiy Tesuvah] however hold that one goes after the time zone of the area that he is in. Thus if one traveled from Israel to America then one can sell it before the 6th hour enters in his area America, and does not have to precede it to the time in Israel.
The Final Ruling: Practically we rule like this latter opinion, although its proper to be stringent like the former opinion to get rid of the Chametz prior to the 6th hour in the area that the Chametz is in. [So rules Igros Moshe 4/94. Minchas Yitzchak 7/25 brought in Piskeiy]
 Accordingly when one has traveled to a later time zone [east to west] one would need to make 2 sale contracts, writing in each one that it is only valid for the Chametz of its area, in order so he be able to still eat Chametz in the west after the time of the East has arrived.
 Pashut. Brought in Blumenkrantz
 Piskeiy Teshuvah
 Igros Moshe 4/94
 Rather the Rav is to buy it for himself and then the original owner will buy it back from the Rav.
 Yagdil Torah Yerushalyim 6/2610
 Rav Avraham Chayim Naah rules that one may use the Chametz immediately after Havdalah even if the Chametz was not yet bought back and there is no problem here of stealing or of any other prohibition, as the gentile does not care if one does this”. And this may even be done lechatchilah if one writes in the contract the above condition. And so should be done. See Shaar Halacha Uminhag 2/195