Appointing an agent through an electronic medium
There is no absolute requirement that the appointing of an agent be done through a physical act of "kinyan" or in the presence of witnesses - even private verbal instruction is sufficient (Shulchan Aruch, Choshen Mishpat #182; 1). One can even appoint an agent without his knowledge; even by way of a written "harsha'ah" contract. See "Seder Mechiras Chometz" of the Shulchan Aruch HaRav (Shaar Hakollel, Note 38):
"Someone who wants to appoint an agent without his knowledge in order to sell chometz should write as follows: …"
"Signing" a harsha'ah contract via the internet is actually not significantly different from signing a harsha'ah without the knowledge of your agent, and, thus, it works. See also Teshuvas R' Shmuel di Medina (Shu"t Maharashdam, Choshen Mishpat, #146) who brings the words of the Rambam and the Tur who wrote that appointing an agent does not require a formal transfer (kinyan) but that the privately spoken word is sufficient. The Maharashdam then adds,
"if so, since the power of a written command is surely greater than of speech, and once a person writes to the agent in his own handwriting, everything that he wrote is binding. And all the more so if it is the way of merchants to complete transactions by way of such written instructions…From this we can learn that one who instructs his fellow (in writing) to do such and such, and the fellow does it, the act is valid and it's impossible for someone to (later on) raise any claim against his agent."
So signing an electronic harsha'ah via the Internet (which has become a universally accepted medium of commerce), should certainly be considered equal to a signature and, if it works for regular commercial dealings, it should similarly work for selling chometz.