Selling is a state of being and an absolute must. You have to sell your product yourself at any time. You should accept that, first.
Selling is about creating awareness and about meeting targets. When you are selling early on, you are definitely not meeting targets – you are merely spreading the word and finding few believers in your idea. You sell to find a co-founder, you sell to find your first few hires and you sell to find your investor. Only then, you sell to find a customer.
Selling starts early: you practically sell from the beginning. Who decides about your product’s viability? Your customers. Unless they use your product, don’t talk about having a MVP! Show them your product, roll out an unfinished version (as you can read in every Lean Startup book). Keep iterating until your customers like what they use. Don’t do that in isolation!
Selling is YOUR job. Yes, indeed. You have to sell by yourself (don’t delegate sales to a “sales guy”).