The Talmud relates a story of a man travelling on a boat with chameitz. Realizing that he would be stuck on a boat over Pesach with a large quantity of chameitz, he decided to sell it to a non-Jew who was travelling with him. After Pesach, he bought the chameitz back. Over time, it was common practice for businesses that dealt with large quantities of chameitz would sell their chameitz to a local non-Jew to save them from incurring a large financial loss. Nowadays, in an age of stockpiling, it would be a substantial loss for any of us to just get rid of all chameitz products before Pesach (especially after Purim!), we therefore sell our chameitz to a non-Jew for the duration of Pesach.
The sale itself is rather complex due to the fact that there is no transfer of the physical items being sold. It is therefore customary to appoint a competent rabbi as one’s agent to sell their chameitz.
All chameitz food that is being sold must be placed in an enclosed area. If one could place it in a separate room that is most ideal. If not, cover in a way that makes it difficult to access. If food is in a cupboard one should tape the cupboard down.
We do not sell our pots and pans that have been used for chameitz The reason for this is that if we were to sell a non-Jew our dishes and buy them back after Pesach, we would have to tovel all of those items that we just bought from a non-Jew.
If one will be in a time zone that is different than the time zone that the sale will take place in, one should discuss this with their rabbi. There are some complications with such a sale. For example, if one is in California, where they are three hours later than us in Baltimore, and the rabbi buys back the chameitz immediately after Pesach, the individual who is in California will be in possession of chameitz for the last few hours of Pesach.
One need not line cupboard shelves or drawers with shelving paper. Since they do not come into contact with hot food the only concern is cleaning the area out from any Chameitz.
One may use towels and table-cloths from year round on Pesach as long they are washed in hot water and soap before Pesach.
(Note that I want to emphasize that I am writing what is necessary by law. Jewish People from time immemorial have cleaned for Pesach with extra stringencies and it is a beautiful testimony to our ancestor’s commitment to G-d. I would advise that if one has a custom that is a stringency one should treasure it and hold on to it. If for some reason it becomes difficult to maintain, it should be weighed seriously before being discarded.)
If something is inedible we do not treat it as Chameitz. However, if one puts that item in their mouth to digest it the Poskim debate if in doing so one gives the item the status of food. In that light I would recommend the following:
Medicine: Bitter or tasteless tablets, capsules, or liquids may be taken on Pesach even though they have chameitz ingredients. (If the medicine taken is in the form of pleasant tasting tablets or liquids an alternative should be found. If none is available, check in with a rabbi before discontinuing use.)
Vitamins: Ideally, a chameitz-free vitamin should be used. If none are available, then as long as the vitamin is bitter tasting or tasteless and prescribed by a doctor, the vitamins may be taken on Pesach.
Eye drops, ear drops, nose drops, and throat sprays do not need to be chameitz free.
Wrapping it up…
A few more things that do and do not need to be kosher for Pesach: Paper-ware and plastic-ware do not need to be certified as kosher for Pesach. (However, some do not use paper plates unless it is certified to be Kosher for Pesach. This is due to the fact that corn starch is used in the plate.)
Dish soap, strictly speaking, does not need to be kosher for Pesach but the prevalent custom is to use only kosher for Pesach dish soap.
Liquid cosmetics that contain alcohol, wheatgerm, or vitamin E, is a matter of debate. One may rely on those that permit using perfume, cologne, shaving lotion etc. on Pesach.
One last thing, although not typically eaten by humans but still must be kosher for pesach is pet food. (Pets may be fed food with kitniyot.)