I'd focus talking to potential customers and building out whatever features make the product "minimally viable" for them. If you're pretty sure you know what they need out of your product (which, unless you have credit card numbers, you probably shouldn't be), get your payment system sorted, throw Google Analytics in, and launch.

At that point you can start selling, selling, and selling some more. Dan Shipper wrote a great post about this today: http://danshipper.com/nothing-happens-until-the-sale-is-made