I think hackers steal other people's credit cards info and buy loads of keys with them to sell on. People find out and get a refund from their bank so the devs/ pubs receive no money, but the keys are already sold and people have the game. I'm not sure if stuff like this has been cracked down on yet. Anyway, for reasons other people have said and more these sites are just bad.i checked and while most resell sites do this CDKeys does not. they do region exploits in which they buy the keys in a low cost region and sell them in a high cost region.
not "illegal" nessisarily. but for me,.. not moral enough.