This is the approved revision of this page, as well as being the most recent.
Institution of Selling Chametz
- Some authorities question the sale of complete Chametz, however, most authorities allow it and such is the minhag. 
- A store shouldn't buy non-perishable chametz before Pesach, then sell them to a non-Jew, so that he can have them immediately after Pesach and sell them.
- There’s no reason or special practice to sell Chametz if one can eat all one’s Chametz. 
Those who don't sell real Chametz
- Even if one wants to observe this stringency, it is advisable to perform Mechirat Chametz for mixtures of Chametz. 
- If one doesn’t sell real Chametz such as cake, bread, or noodles, one may still sell ketchup, mayonnaise or any mixture of a minority of chametz. 
- If one doesn’t sell real Chametz such as cake, bread, or noodles, one may still sell flour commonly found in the market which is Chametz since in the processing the grain is washed and comes into contact with water. 
Procedure of Selling Chametz
- When selling Chametz one should write down one’s address and, if one is going away for Pesach, provide access to get in by leaving the keys by the Rabbi, neighbors, or security guard. If one feels uncomfortable doing that one should at least write down one’s cell phone number at which one would be reachable over Pesach.
- A person should appoint the rabbi to sell his chametz by the seller picking up a handkerchief of another item of the rabbi. If that can not be done, the seller should sign the authorization document. In cases of great need he can appoint the rabbi orally over the phone.
- One should make sure to sell one's chametz completely and not with a gift with a condition that it must be returned after Pesach. However, one may assure the non-Jew that one will buy it back after Pesach and make it profitable for the non-Jew.
- One who sells his Chametz, should not sell the containers which are holding the Chametz to the goy because if he does so, he may be required to dip them in the Mikveh again. If one did so, he should dip them in the mikveh after Pesach, without a beracha.
- One may not sell one's chametz to a Jew who "converted" to another religion.
Stocks of companies which own Chametz
- Some authorities hold that there's is no prohibition of owning Chametz to a shareholder of a company which owns Chametz on Pesach who has no say in the actions of the company, while others hold that one should sell the shares together with one's other Mechirat Chametz through the Rabbi. 
Storing the Chametz
- Chametz which one sells to a non-Jew for Pesach should be put away in a closet or another partition of 10 Tefachim and not opened on Pesach. Similarly, one shouldn’t leave sold Chametz out in the refrigerator but rather it should be partitioned off with a wall of ten Tefachim. 
Seventh Day on a Friday
- If the seventh day of Pesach falls out on a Friday in Israel, some poskim say that that an Israeli can eat Chametz on that Shabbat and the Chametz isn't considered muktzeh because of the previous day.  Others are strict. 
Bedikat Chametz in Rooms That Are Sold
- Yalkut Yosef Hilchot Pesach vol. 1 (Hebrew, 5775)
- Mechirat Chametz by Rabbi Mordechai Willig
- Article on Mechirat Chametz: Theory and Practice by Rabbi Josh Flug
- The institution of Mechirat Chametz is based on a Tosefta (Pesachim 2:6) that says that a Jew on a boat with a non-Jew may sell his Chametz to the non-Jew and then buy it back after Pesach. This is codified by the Rambam (Pesach 4:6) and Shulchan Aruch 448:3. The Beit Yosef 448:3 writes that selling it before Pesach and buying it back is Ha’arama (a scheme to avoid a prohibition) and yet it is permitted. The Talmid Haritva printed at the end of the Chiddushei Haritva on Pesachim limits this permission to an incidental sale of the Chametz but doesn't allow the sale in an institutionalized manner.
- Gemara Shabbat 139b says that a certain Rabbi slept on a non-Jew’s boat on Shabbat claiming that he intended to sleep, even though he knew the non-Jew would sail the boat across the river. The Gemara says that his claim was HaAramah, but is permitted since it’s only an rabbinic prohibition and he was a talmid chacham. Bechor Shor (Pesachim 21a) learns from here that Ha’aramah is permitted only if the issue is derabbanan. He writes that after Bitul, the issue of owning Chametz is only derabbanan and Ha’aramah is acceptable. Bet Efraim 1:33 and Minchat Bikkurim (Tosefta 2:7) agree. Yeshuot Yakov 448:9 theoretically agrees with the Tevuot Shor but in practice forbade selling chametz to a non-Jew to sell to one's animals because of haarama.
- Mekor Chaim 448:11 rejects the Bechor Shor and explains that in Gemara Shabbat HaAramah was only permitted on an isser derabbanan because the Rabbi was actually violating an prohibition, however, by selling Chametz, one circumvents the issue altogether.
- Rav Yosef Dov Soloveitchik (quoted by Rav Hershel Schachter in Nefesh HaRav p. 177) held that one shouldn’t sell Chametz BeEyn, actual chametz. [Halachos of Pesach (pg 32) defines Chametz BeEyn as mixtures which have a majority or equal amount of Chametz in comparison to the non-Chametz ingredients.] This opinion is based on the idea that Ha’aramah shouldn’t be performed except for Derabbanan issues. According to this, one may rely on Rabbeinu Tam (Pesachim 42a s.v. VeElu), who holds that mixtures of Chametz are only a derabbanan issue of Bal Yeira’eh.
- Rav Schachter in BeIkvei HaTzon (siman 15) writes to defend the minhag that Ha’aramah is acceptable to avoid an isser, but not to exempt one from a mitzvah because at the end of the day, one didn’t fulfill the mitzvah. Therefore, selling one’s Chametz only avoids the isser, and one may still fulfill the mitzvah of Tashbitu with burning the last unsold kezayit.
- Halachos of Pesach (p. 123) records the practice of Rav Aharon Kotler not to sell Chametz BeEyn. However, Halachos of Pesach quoting Rav Moshe Feinstein, Tzitz Eliezer 20:51(2), and Yalkut Yosef 448:3(1) permit selling all forms of Chametz. A Guide to Practical Halacha v. 5 p. 66 n. 5 also quotes Rav Moshe to this effect, but the Debrecener held one should try to use up one's chametz before Pesach. Halachos of Pesach (p. 123) writes that most poskim allow the sale and that such is the minhag. See, however, Haggadah of the Roshei Yeshiva (pg 13) which writes that Rav Moshe's minhag was not to sell actual chametz but he did sell alcoholic beverages.
- Yalkut Yosef 448:3:9 writes that even though the minhag is to use the sale of Chametz a store shouldn't initially buy chametz in order to sell it.
- Rabbi Sobolofsky at yutorah.org between minutes 12 and 13
- Halachos of Pesach (p. 123) writes that sometimes those who desired not to sell Chametz unknowingly retained Chametz items. He adds that a student in a dorm should not assume that his Chametz was sold for him; rather, he should specifically request his father (or another individual) to be his representative to sell his Chametz. http://www.torah.org/advanced/weekly-halacha/5757/metzorah.html also says that one should sell his chametz even if he doesn't think he has any because there may be denatured ethyl alcohol in deodorant, shaving cream, or cologne.
- Rav Schachter on OU Pre-Pesach Webcast 5771 between minutes 26 and 27
- Rav Yisrael Belsky on OU Pre-Pesach Webcast 5771 between minutes 79 and 80
- Rabbi Sobolofsky at yutorah.org between minutes 4 and 6
- Rav Moshe Feinstein (cited by A Guide to Practical Halacha v. 5 p. 65 n. 4)
- Shulchan Aruch 448:3 writes that one may not do a Matana Al Menat LeHachzir. Kitzur Shulchan Aruch 114:6 agrees. Mishna Brurah 448:21 explains that even though usually a Matana Al Menat Lehachzir works to completely acquire something (such as Shulchan Aruch 558:4), by chametz because of the chumra of chametz one may not do a Matana Al Menat LeHachzir.
- Kitzur Shulchan Aruch 114:6, Mishna Brurah 448:23
- Chachmat Adam 73:3, Yechave Daat 3:24
- Yechave Daat 3:24, Nitai Gavriel Pesach 1:44:8
- Kitzur Shulchan Aruch 114:7
- Piskei Teshuvot 440:1 and http://www.vbm-torah.org/pesach/stocks.htm quote the dispute. Rabbi Yisrael Belsky based on Rav Moshe and many others at the OU Pre-Pesach Webcast (5770 min 29-32) permits someone who owns shares but doesn't have any say in the company, however if someone has 30% (or so) of the company it's considered as if one is a partner and it'd be forbidden. This is implied by Igrot Moshe EH 1:7. Rav Dovid Feinstein (cited by Journal of Halacha and Contemporary Society v. 24 p. 85) agreed that it is permitted to own stocks of a chametz company if one doesn't have any involvement in the voting or management of the company. Haelef Lecha Shlomo OC 238 agrees. Minchat Yitzchak 3:1 and Moadim Uzmanim 3:269 say that one should sell these stocks along with the rest of the chametz and not trade them during chol hamoed.
- Rabbi Sobolofsky at yutorah.org between minutes 6 and 8.
see Rav Yisrael Belsky (OU Pre-Pesach Webcast 5771 between minutes 27:00 and 28:30) says that one shouldn’t store Chametz that is sold on the shelves of the door of the refrigerator. Instead, it should be put in the back of the refrigerator and sealed off.
Rav Meir Mazuz (Hashem Nisi part 1 pg. 13 and Ohr Torah 5749 Siman 101) writes that it is not appropriate to sell Chametz and then block it off with just a piece of paper right next to the food that is Kosher for Pesach because that makes the sale seem like a joke. Instead, one should lock it off in in the refrigerator or cabinets.
- Yechave Daat 2:64, Yalkut Yosef 448:5 citing Yabia Omer 9:46
- Or Litzion 3:9:4