I have seen online forms and received emails from many far-flung places offering to sell my chometz for me. I also have a competent Rabbi in my local shul who is willing to do it. No two of these chometz-sale forms seem to be alike and I am not competent enough to know if one is better than another.
My question is is there any reason to prefer selling chometz through a local reseller as opposed to anyone else?
It's customary to appoint the rabbi your agent to do effect the sale using a belt-and-suspenders approach: a signed contract, an oral authorization, and a sudar. Is it necessary? No, which is why some respectable rabbis accept agency via the Internet. But it's customary. And sudar (and possibly a signed contract, too) is only done in person, not online.
A Web page seemingly by Rabbi Chaim Jachter and another (see section Ⅲ) seemingly by Rabbi Gil Student cite further sources re the belt and suspenders.
In addition to msh210's point that it is preferable to perform a physical kinyan when appointing a Rov to sell your chometz, there are additional factors that should be taken into consideration when choosing who to sell your chometz through.
Chometz should ideally be sold in the same time zone where the chometz and person will be during Pesach. If not, the chometz may only be sold after Pesach starts, or be bought back before Pesach ends for the person in the other time zone. When travelling, and the person will be in a different place to his chometz, a Rabbi should be consulted.
Additionally, broadly speaking there are two Halachic methods utilised in the sale. The Noda Beyehuda established a method known as "זקף עליו במלוה". Later the Baal Hatanya noted that based on Rishonim who were not available during the time of the Noda Beyehuda, this method is highly problematic, and he publicised an alternative method called "ערב קבלן". Most authorities agree that this type of sale is preferable, and some would even invalidate the former way of selling chometz.
