The real issue for these non-sellable airline items is that most can be legally "given away," just not sold.
Clearly, the white envelope is nothing more than a charade.
However, what if the item in question came days apart (let's say an upgrade cert). Meaning, the owner sends the white envelope to the winning bidder upon payment. They strike up a 3 or 4 day e-mail dialogue to become "friends," and then the cert owner gifts the cert to his/her new friend.
Certainly, that skirts the rules; but at some point, it's probably not worth it for the airlines to pursue it over a $75 transaction.