There's nothing wrong with selling your product ahead of time. You just have to be absolutely sure that it will be produced and delivered in the expected time frame. Your consumers will expect that you meet the delivery date if they've paid in advance and if for some reason you fall short of that, you will lose the confidence and the trust of your buyer not where you'd want to be with a new business.

So as long as you can secure the delivery date, then make every effort to boost pre-sales and market the product to do so! The iPhone is a great example of a product that pre-sells for weeks. Many people hear about the product or spend time talking/writing about the product, which in turn creates quite the buzz. Really emphasize the fact that you are pre-selling your product too to create your own buzz!