As a web design shop we would like to license Craft not for ourselves but for our customers who run and own the future websites. How do we do that?
A Craft purchase/edition is tied to the
craft/config/license.key file that is in place at the time a purchase is made and wherever that file goes, the purchase/edition will follow.
In addition to that, a
license.key file can only ever be tied to one public domain at a time.
As long as you give your client that
license.key file and don't use it for other clients or projects, they "own" it.
How the license gets paid for is more up to you and your client. i.e. either you purchase it and bill the client as a line-item on an invoice or have them purchase it directly.
Some relevant reading: