"How to sell hosting?" - Simple, sell service. When you sell service, the hosting will sell itself.
And you're already on the wrong foot if your calculating how many packages you can sell into your XX GB's of hdd and XX GB's of transfer. To provide good service you need to balance based on avg CPU loads... Storage and bandwidth are secondary.
You can fit 100's of clients on a box when all or 99% of those clients are utilizing little/no CPU and they'll pretty much all get responsive service. However, get a few heavy hitters on there and you've just ruined the service for all clients on that box.
If you're going to start with one box, make sure you watch the accounts you put on there, and know when to start selling into another box, even if your jumping off your first box with a monthly loss (i.e. oversold to heavy hitters, not enough account income to cover cost of lease). Otherwise, you'll get branded as crap before you ever get off the ground.
There's more to hosting than just throwing up a box and gaining accounts, you have to be a sysadm and maintain a reasonable system load. Too low, and the box is underutilized, you're losing money. Too high, and the service will suffer.
Good luck with your venture,
Mike