[schmrz]

While it may not be "right", you don't need an inventory, not even a payment system. You will not be charging the potential customers. The point is that you test the market, just to see if anyone would be interested in buying. Once they get to the checkout, simply say that it will be "available soon" and optionally ask for email so you can actually sell to them later.

For anything you want to build and sell, you need to make sure that you (a) have a market, (b) can reach the market (marketing).

Again, it may not be completely right, as you may feel (as I do) that you are misleading customers a bit.