American Jews take Pesach very seriously. In fact, according to the Pew Research Center, 70% of American Jews participate in a Seder - which is more than the number who light Shabbat candles, fast on Yom Kippur or keep kosher at home.
We take Pesach very seriously - which is the reason that we not only clean our homes assiduously and nullify any remaining chametz religiously, but in modern times we sell our chametz halachically.
It's not that Jews never sold chametz to a non-Jew before Pesach. But, until the Middle Ages, at Pesach-time Jews would simply remove their chametz from their homes and sell it outright to a non-Jew.
But then, in the Middle Ages, Jews began to operate distilleries which had so much chametz on premises that it could no longer be removed. As a result, the 16th-century Polish sage, Rabbi Yoel Sirkis, known as the Ba"Ch, introduced the novel idea to both sell the chametz and rent the spaces where it was stored, so that the chametz would no longer be controlled by the Jewish owner. This is the very same halachic procedure we follow today, as modern food technologies allow us to store so much chametz in our homes that it is unreasonable to expect Jews to remove everything tht could be sold.
But the sale of chametz and rental of space is not a simple halachic matter, one that a typical layman knows how to do.
Enter the rabbi - who despite common parlance - does not "buy the chametz" before Pesach. Rather, the rabbi serves as the designated agent, shaliach, to arrange for the sale of chametz on behalf of his congregants.
Procedurally, appointing an agent is very simple and can be accomplished by contract or through a symbolic transfer known as kinyan chalifin - the lifting of a handkerchief or other such item. Ideally, this action is done in person, but when necessary can be done by mail or even over the phone.
Therefore, rabbis typically make themselves available to their membership for several weeks before Pesach so that laymen can appoint them as agents, completing a form which details where all of the chametz will be stored. At that time, and to add to the seriousness of this exchange, there is also a custom to pay the rabbi (often a donation to a special fund) for his services.
On Erev Pesach the rabbi will arrange for the sale of all the chametz from all of the people who have assigned him as their agent to a non-Jew. (For those who travel to Israel, and because of the eight-hour time difference, we also conduct a special sale of chametz on the day before.) At the cRc, this is accomplished in the presence of a Beth Din overseen by HaRav Yona Reiss, shlit"a.
First, there is a contract that is written between the buyer (the non-Jew) and the sellers. This contract is a legal document and includes provisions regarding the rental of locations where the chametz is stored, different types of chametz, chametz that may have been missed, and even when the sellers may be in a different time zone the buyers.
Next there is a transfer of money, a down payment for the rented spaces, and the chametz which will be fully appraised following the conclusion of the holiday. This is followed by a kinyan chalifin, the exchange of an item such as a handkerchief; a handshake, tekiat kaf; a verbal agreement, kinyan odita; and the rental of space through which the moveable property (the chametz) can also be acquired.
All of these steps are safeguards, just in case the ownership of the chametz may not be adequately transferred, and the process while well-practiced and detailed, requires great expertise and experience.
But it's all real, and it's all 100% legal. So, if during Pesach the non-Jew wanted to get a bowl of chametz cereal from one of the seller's homes - he could! And if after Pesach the non-Jew chose to finish paying for all of the chametz, it would be his.
But that's the way it is supposed to be - because we take Pesach very seriously.