BTW. Humble is giving away steam keys yes... But these keys have to be claimed on Steam accounts, so there is no option for resale.
There may be a warning message on the Humble giveaways and the key will disappear from the page after the stated deadline, but once you have the Steam key in your possession it will generally continue to work even long afterwards and for the foreseeable future. It's rare that the Steam keys themselves actually expire.