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.