I feel the morality behind the potentially buying keys that 'fell' off a truck, but in the end if you buy games on sale your basically giving the devs the finger anyway. but largely there is a big difference between say, buying a key online to buying pirated dvds from someones car boot on one hand you know what your doing is wrong, on the other hand you only know what your doing 'might' be wrong. I had heard that sometimes these key are either giveaways or the company has actually listed them there themselves to try to just get rid of them in bulk, how true that is, is anyones guess.
i think i've only heard this twice now that someone has come forward and said 'hey hold on, someone is selling our stolen keys on g2a'