Forgive me if I'm too obvious here, I mean no offense - just trying to make things clear for you.
Dreamhost is a web host and a registrar. A Registrar reserves domain names - example.com for you. Once you own a domain name you may point it to a host of your choice.
A Web host such as Dreamhost will sell you shared space on a server to host your files.
To have someone be able to type in your domain name - example.com and get the website that you've created you need to both register a domain and host it. Those services can be done through the same company or two different ones.
When you sign up with Dreamhost for a hosting package you're allowed to host unlimited domains and sub-domains - example.com and sub.example.com. You're given x MB of storage and x TB of bandwidth. You can use those allotments however you want - as long as it's within the TOS. Reselling is one thing that you can do with your space - however Dreamhost doesn't exactly cater to that.
If someone is web-savy they'll most likely know that you're re-selling from Dreamhost. That may or may not be an issue for you - depends on what services you're offering. Dreamhost doesn't make things easy or automatic for you to resell though - you'll probably have to do a lot of things behind the scenes to make that happen.
Hope this info is helpful
art.googlies.net - personal website