如何保护iPhone等开发的应用程序免受转售

I have developed an Application for iPhone. In some countries, there are some places you can go, give your iDevice and they will install any App/Apps on it much, much cheaper(for 1$ for any App). How do they do that? - They simply legally buy an Application once and then synchronize "customer's" iDevice with there Account. I think it's illegal, to resell... I have developed an Application for iPhone. In