All reseller accounts are setup with a folder named moving.page in your public_html directory.
This folder contains the page your client will see if the you move the account to another server via the Express Transfer or use the Transfers functions in WHM. The page will show on the old server while the domain is propagating to notify visitors of your site that the page is currently being moved to a different server.
This page can be customized to suit your business image. In order to edit it, download the index.html file contained in this folder, modify it with your favorite HTML editor (eg: FrontPage or Dreamweaver) and reupload it to the same folder.
Alternatively, you can edit the page's source code directly by logging into WHM and clicking on Web Template Editor under the Account Functions menu and then clicking on the Account Move tab.