Firstly, as you are probably already aware, DreamHost does not use cPanel, they use their own custom admin panel, that (in my opinion) is superior to cPanel and provides far greater functionality. However, it is different, and this difference can be the source of much frustration for those moving from a cPanel based host.
Each domain or sub-domain can be placed under the control of a different user and each of these users has a separate home directory that contains (amongst other things) the web accessable directories for any domains they control. When a particular user logs-in via FTP, they will see only the files for the domains they control. You can set disk space quotas for each user, but as far as i am aware, you can not set traffic quotas on a per user basis.
You can also create new WEBIDs (panel log-ins) and grant these WEBIDs panel 'privileges', which control what they can do in the panel. One thing you can not do is 'brand' the panel, your clients will always be aware that they are hosted at DreamHost.
Overall, DreamHost is a damn fine host and offers more than most people need in a host, but it may not be the best choice for a re-seller.
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