Every Jew, before selling ḥametz, should read the authorization contract he will be signing, so that he understands that he is empowering the rabbi to sell his ḥametz, and that the sale is absolute. But even if instead of reading the contract one simply relied on the rabbi, the sale is valid, for, if the gentile comes during Pesaḥ to take the ḥametz, and the rabbi tells the Jew that the ḥametz indeed belongs to the gentile, and that he must give it to the gentile, he will do so.
The seller should write his name and address in clear script on the contract of sale so that the gentile buyer knows who he is and where he lives. Thus, the gentile buyer will be able, if he wants, to collect his ḥametz.
Le-khatḥila, it is proper to give the gentile the key to the place where the ḥametz is located so that he can enter and take his ḥametz. In practice, though, it is sufficient to give the gentile the seller’s phone number so that if the gentile wants, he can call to come and take his ḥametz. This is the most important principle: all sellers must know that after the sale, the ḥametz indeed belongs to the gentile, and the seller must allow him to enter the house and take his ḥametz (MB 448:12).
Le-khatḥila, the seller should indicate the various types of ḥametz, and even list their prices, on the contract the rabbi gives him to sign, and some people are meticulous about this. In practice, though, this is very difficult to carry out. Therefore, the custom is to write that all ḥametz in one’s possession is included in the sale, and that the price is in accordance with the accepted market price, as determined by appraisers (see BHL 448:3, s.v. “be-davar mu’at”).
It is proper to write in the contract where exactly the ḥametz is located, for example: “In the upper left kitchen cupboard,” or “in the room on the right, in the box so marked.” Several places may be listed. Be-di’avad, even without this, the sale is still valid, though all ḥametz should be gathered to one place and labeled. Everything gathered to the place determined for the sale by two hours before the onset of the prohibition against deriving benefit from ḥametz is included in the sale. Le-khatḥila, the place where the ḥametz is kept is also rented to the gentile, so that the ḥametz is in the gentile’s possession and the sale resembles any other sale in which the buyer transfers the purchase into his possession.
One may sell ḥametz through a proxy who writes all relevant details and signs on the owner’s behalf. One may also sell ḥametz over the phone, by fax, or via the Internet. Even though it is proper for the seller to sign and perform a kinyan (act that formalizes a transaction) to reinforce the sale, be-di’avad it does not prevent the sale from taking effect, as the most important thing is the kinyan performed when the rabbi sells the ḥametz to the gentile, and this kinyan is effective for everyone who asked the rabbi to sell their ḥametz.
In kitchens that belong to public institutions, the manager or his representative sells the ḥametz.
One should not sell the ḥametz that is stuck to and absorbed into kelim. Quite a few laws relating to mekhirat ḥametz were introduced in order to make it clear to all that it is an actual sale, but if one writes that he is selling the ḥametz absorbed in or stuck to his kelim, the sale will appear to be lacking seriousness, since this has no value and nobody is interested in buying it. Therefore, one should not indicate this in the sale contract.
See Sidur Pesaḥ Ke-hilkhato 11:7-8 and in the notes. Ibid. 12 states that one is permitted to enter into a ḥametz sale agreement with multiple rabbis. Even if each rabbi sells to a different gentile, the sale completed first takes effect. Preferably, one should not do this, since it makes the sale seem fictitious. Only if he is worried that one rabbi may not sell his ḥametz may he ensure that his ḥametz is sold one way or another. See Piskei Teshuvot 448:6.
. Selling the kelim themselves to a gentile is not worthwhile, since after Pesaḥ one would be required to immerse the kelim in a mikveh, like any eating implement bought from a gentile. This is the view of Ḥatam Sofer OḤ §109 and most poskim. (However, AHS YD 108:52 is lenient about this.) The truth is, though, that this is unnecessary, because once the ḥametz was nullified, there is no cause for concern. Even if there is actual ḥametz substance on the kli, as long as there is less than a kezayit cumulatively, according to most poskim there is no need to eliminate it, as explained in SAH 446, Kuntrus Aḥaron 1 (above, 4:5). However, one must store these ḥametz utensils in an enclosed area, and after Pesaḥ one must remove any remaining ḥametz substance before using them. Le-khatḥila, in order to satisfy the opinion that one is required to eliminate even less than a kezayit of ḥametz (MA 442:12), one should remove any trace of ḥametz from the kelim before Pesaḥ or ruin the residue with detergents until they are no longer fit for a dog’s consumption. In this case, even if more than a kezayit remains, there is no need to eliminate it. Therefore, there is no need to sell the ḥametz on kelim, and certainly not the ḥametz that is absorbed in the kelim.