You need not to, you can very well be a seller of a mobile app in individual capacity, you should go to register yourself as a company (developing & selling apps) only when you've a regular team that needs salaries, or your investors specifically want you to do so.

In initial stages, all you need is an app developer time like the one we offer (if you don't develop yourself), a developer account on itunes and google play (so that you can host the app in your name), and a bank account attached to those store accounts.

It is not much difficult to migrate your individual store accounts to enterprise one once you've a company ready in future, so don't worry about these things and focus completely on development, marketing, and user retention of your app project.

So far as type of company is concerned, you can settle for LLC and keep it lean until you change in future for Inc.