I think this is against Humble TOS. You should double check.
I have checked the Humble TOS and could not find anything to say it's illegal.
Ah, I don't want to step on any toes here, but in section (xii) of the ToS it does mention that you are not permitted to engage in: "...distributing, transmitting, copying (other than re-installing software or files previously purchased by you through the Service on computers, mobile or tablet devices owned by you, or creating backup copies of such software or files for your own personal use) or otherwise exploiting the Products (defined below) in any manner other than for your own private, non-commercial, personal use." Readable here
Trading your Humble Bundle credentials away to someone else is most likely in violation of these terms, since it no longer falls under re-installing or backing up the game files to your personal devices.