Irene's right dawg97, it would be of some difficulty to find someone who would be willing to disclose their earnings. And you yourself said that it depends on many factors..
And it does, so it would be hard and unfair to give you even an average amount. Monthly sales these days are just too erratic, location, currency exchange rates, shipping, marketplace, customer needs and supplier types are among the many factors that can affect a seller's monthly income. Once person's average may not necessarily be that of another.