I think if you have good sales (or sales oriented) people on the team, you probably already are selling before you build, regardless on what stage the company is in.
As developers, we'd often complain about sales people selling features we haven't yet created. Only after learning about Steve Blank's Customer Development approach I realized that's how it's supposed to be. Sales people are the ones in contact with the client, and incoming revenue is the signal the real world is giving you on the value of your product.
To be fair, developers complain about salespeople selling features that don't exist yet and promising delivery by a certain date without consulting us. That kind of thing makes our lives hell and makes us write terrible crap that undermines all of the company's software assets.
Selling features that don't exist is ok, just check with the guys that have to deliver on your promises first.
I know what you mean, I've been the developer in this scenario more than once :)
In the end it's a matter of balance between sales and development. Unlike the common "10 things you must do" stories on HN, there aren't really any well-defined rules for this. You just hope to have smart sales people and developers, good communication between them, and have neither side controlling the company.
As developers, we'd often complain about sales people selling features we haven't yet created. Only after learning about Steve Blank's Customer Development approach I realized that's how it's supposed to be. Sales people are the ones in contact with the client, and incoming revenue is the signal the real world is giving you on the value of your product.