What is Horav Miller’s Shlit’a opinion of selling one’s Chometz via the internet as is very common in our days?

Although in principle the naming of the Rabbi as an agent to sell may be done via the internet. However, it is probably better to use personally your local Rabbi if you can, for an important reason. The relationship between a congregant and his Rabbi needs to be a strong one in order to grow in Torah and be part of the shul. If you make the effort to meet your Rabbi and use him for your Halacha questions and other ritual needs, you will get close to him and enrich your Torah and experience greatly.