You don't have to actually have a product before you sell it, or build something to see if it will sell... It's called vaporware : a computer-related product that has been widely advertised but has not and may never become available
It's called "Minimum Viable Product" now, but 15 years ago we called it vaporware.
http://en.wikipedia.org/wiki/Minimum_viable_product
The canonical MVP strategy for a web application is to create a mock website for the product and purchase online advertising to direct traffic to the site. The mock website may consist of a marketing landing page with a link for more information or purchase. The link is not connected to a purchasing system, instead clicks are recorded and measure customer interest.
That is how you get sales before you have a product... Or at least know that you'll get sales.