I see that many Jewish sites make Mechirat Chametz on the Internet. I know that traditionally, people make the sale by writing a Shtar and using a Kinyan Sudar to make the Rav a Shaliach to sell the Chametz. How does the Qinyan work when somebody only writes his name and address on an Internet site?
I also don't understand why a Qinyan is needed to make the Rav a Shaliach. In several Gemarot we see the concept of "Qanu Miyado", or "Qanu Mineh", e.g. in Sanhedrin, to give the power of a Bet Din which is not regular, or in Baba Batra for Matnat Shchiv Mera. Rashi explains in BB that it's a Qinyan Sudar. The Qinyan Sudar is called Chalipin and we rule that it's made with an Utensil belonging to the purchaser. In the Qetuba, the Chatan makes a Qinyan Sudar for the commitment of the Ketuba. It's not clear to me what is the source that a Qinyan Sudar is needed to appoint a Shaliach, even for Kitvu Utnu Get.