The trinket seller? No, I don't know their real name either. Even I don't know a lot about them - and I know a lot.
They can only say so many words per day. The quiet isn't entirely a choice. I think Darroch knows why, but he's certainly not saying anything.
The key to the mystery door bothers me. Did you pick it up? If I ever find out where it goes, I'll tell you for free - since you didn't exactly get your coin's worth out of this secret.