Reseller accounts were created to allow web designers to offer hosting services to their customers so that they can become a one-stop-shop for all their web design and hosting needs.  However, you don't need to be a reseller to buy a reseller hosting plan. 

Reseller accounts are usually slightly more expensive than regular hosting accounts, but they will allow you to many different domain names without incurring in the add-on domain fees, making more efficient use of your space and bandwidth.

You want to choose a reseller account that offers:

• no limit to the number of domains you can add.
  
  
• the possibility of hosting your main website free, without using any of the space and bandwidth in your hosting plan. If you are a reseller, your main website will be your business website, and the space and bandwidth will be reserved for your customers. If you are not a reseller and just want to manage different websites, you can designate one of your domains as your main website (Tip: select the largest website and/or the one with most traffic).
  
  
• the possibility of upgrading to another plan without any penalties or fees.

To find out how much space and bandwidth you will need, first determine how many sites you are likely to host, how many pages each site will have, and how many visits will those sites most likely receive.  If you plan to host 10 sites with 20 pages each, at an average of 50KB per page and 100 page views per site per day, you will need 10MB of space (10 x 20 x 50KB) and 1.5GB of bandwidth (also known as data transfer rate) per month (10 x 100 x 50KB x 30).  

As far as pricing is concerned, there are reseller accounts costing as little as $10 / month up to several hundreds (or even thousands).  It all depends on your requirements and the hosting company you choose, so shop around to get the best possible deal in terms of cost and reliability.