When people come to the Rabbi before Pesah to appoint him as their agent to sell their Hametz to a gentile, they often conduct an official "Kinyan," a symbolic act to formalize the agreement. After the individual fills out the form and signs his name, he usually either shakes the Rabbi’s hand or takes hold of a handkerchief or some other object, as a formal act whereby the Rabbi’s appointment becomes official.
However, Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his Or Le’sion (vol. 3, 9:2; listen to audio recording for precise citation), rules that this formal act of "Kinyan" is not necessary. It suffices for the individual to express his desire to appoint the Rabbi as his agent to sell the Hametz, even by telephone, and no formal act is required.
An interesting question arises in a case of wine which is not certified Kosher for Pesah and one therefore sells to a gentile along with his Hametz. Once the wine enters the legal possession of the gentile, it should seemingly come under the prohibition of "Stam Yenam" – wine owned by a gentile. It is forbidden to partake of a gentile’s wine, even after purchasing it from him. Must a Jew therefore discard the wine sold to a gentile along with his Hametz before Pesah, as it had come under the ownership of a gentile during Pesah? At first glance, it would seem that even though the wine is permissible as far as the laws of Hametz and Pesah are concerned, it is forbidden because it has been owned by a gentile.
Hacham Ben Sion rules (listen to audio recording for precise citation) that the wine does not become forbidden in this case. The Sages forbade "Stam Yenam" because the gentile had likely handled the wine in his possession. In the case of Mechirat Hametz (the sale of Hametz), however, the wine remains in the Jew’s home and it is highly unlikely that the gentile who purchased the Hametz ever saw it or handled it at any point. Therefore, since we have no reason to suspect that the gentile may have handled the wine, it remains permissible despite having been owned by a gentile during Pesah.
Summary: Some have the custom when appointing the Rabbi to sell the Hametz to perform a formal act of "Kinyan" to formalize the appointment, though strictly speaking this is not necessary. Wine which one sold to a gentile before Pesah along with his Hametz is permissible once it is repurchased from the gentile after Pesah.
