Where is the Mysterious Trader from The Witcher 2 DLC 1?
More specifically, the one from Act 3.
According to this answer to a related question: Flotsam (assuming we're talking about the same DLC).
He was on the porch on the left if looking towards the town from where you entered originally. He sold an improved silversword and some other stuff.
he's near the boats and below where Loredo lives in Act I.
at Vergen, I don't really know how to describe the area, but I saw him a lot when going up to visit Philippa from outside the town. he's also near the prostitutes' camp in the same act. you can access him at both locations, no matter what decision you made in Act I.
I didn't see him in Act III.