Long answer, once used, the CD Key cannot be reused, therefore, selling it would be the equivalent of fraud. Unopened is a whole other story of course.
Yes, there're people selling the cd-keys. Is it ok to do so? Depend on if the key had been used or not. If the key has never been use then it should be ok, if it's already been used then, it's not ok.
If a CD key is unused then there's no problem selling it on. I sold one of my copies of factions to a friend. It was given to me as a present and I had no need of two accounts so sold it to them for a very cheap price.
The only thing Anet frowns upon is selling accounts that are already set up. That is against the user agreement, but as far as I know selling a CD key that hasn't been used yet isn't wrong.
That said however, most ebay buyers are unlikely to buy just a CD key when they can get one off Anet themselves. If they're looking for a bargain most of them will want proof of it not being used, for instance in the original, unopened packaging.
So, while it isn't agains any rules to sell an unused key (as far as I know), any ebay buyer with half a brain would be loathe to buy just a CD key as there's no way of telling if it has been used or not.