"I don't think an author should try specifically to write a series (or a stand-alone) due to any idea of the market.  A concept either cries out to be written over the course of several books with a story arc big enough to support it—some threads that tie up satisfyingly in each book with others that demand more time and effort to resolve—or it doesn't.  Writing two books where one is called for or three where a duology would do only means there's a sagging middle somewhere."