Transfer license keys
Generally, a theme/plugin purchase not only issues a license, but it also starts an annual subscription. Because subscriptions are tied to personal financial information, these types of purchases cannot be transferred between users.
If your developer purchased a license that you now maintain, or you are a developer looking to transfer your license to a client, it's best to have the license/subscription owner cancel the current subscription, which will leave the license key valid until its expiration date. The owner of the site can continue to use the license until it expires. At that point, he or she can then purchase a new license/subscription.
While this is not an actual transfer, it allows both parties to get the most use out of a purchased license. If you would like to go ahead and cancel a current subscription, you can do so on your account page.