Like if someone bought my exchange server off of ebay, can I transfer the licenses to them? Do they have to buy any new cals?
First: Read the EULA - there will be a section in there about transfer of ownership.
In general most software from Microsoft can be transferred to a third party if:
This blog entry puts it better, but is written about Windows 7 - http://blog.superuser.com/2011/04/06/microsoft-licencing-transferring-windows-to-another-computer/