I haven't done this with the blacksmith, and only partially completed the trader but they do seem to follow a pattern. If you catch the trader in the cat of sacrifice, when he dissappears (i don't have a map cuz i'm not in game right now), he goes to the necro that is north of gludio on the left, then the one to the right of that, then pilgrims. Haven't chased him farther than that. Not sure in C4, but in C3, was the same everytime. made it easy to have a high chance of finding him, just check thos for necros in that order.
Just have a lot of alts .. all logged off just near the GK of a cat/necro, then switch between them to see if ppl are buying stuff from inside the gate. Saves you the guesswork and running around.