I guess both eladó and eladónő are correct but eladó is the common one. Simply because when we say eladó, we associate to women. Compare with "nurse". We don't really need to say "female nurse", do we? I guess "titkár" started out being a mostly male profession, hence "titkárnő ". Or who knows why, this is just how it is.
I can hear it a little bit, but it isn't very clearly pronounced. I don't have much experience listening to spoken Hungarian, but from what I have heard it seems as though the gy combination isn't always very clearly pronounced, whereas at other times it's pronunciation is rather clear. I don't know much about pronunciation, though, so perhaps a native can shed more light on this. ^_^