Echoing Mark's point from earlier, here's Spencer Fry:
Make sales or die. A little background on me for anyone who hasn’t been reading my articles since 2009: I started selling on the Internet when I was eleven years old, in 1995. Since then I’ve co-founded three successful companies: TypeFrag, Carbonmade and Uncover. What do they all have in common and why are they all still around? It’s revenue. Revenue comes from sales. If you don’t have sales, you don’t have revenue, and without revenue, you will die.
Nothing much to add. All the successful serial entrepreneurs I know are extremely focused on the concept of sales-first. This is the model that works outside the Valley.
... which makes learning to sell one of the most important things you can do with your time, if this is not part of your skill set.