I did the wedding quest and married off the Merchant to NPC villager. Now they both seem to think that they live in the castle. How do I reset them to the market?

There are a couple ways to fix this, this is just one, with testing cheats enabled.
I am assuming in kingdom mode (not in a quest) that you see the merchant house is unoccupied, if so then create a new merchant (create a sim).
Start a quest with the new merchant. While playing as new merchant find your old merchant on screen, shift key+left mouse click on your old merchant and then select 'add to active family' (as this is your main objective).
Then find and do the same with the wife. Save your game as a new file.
Now, either keep the new merchant as a live-in family member or kill off the new merchant (send them to the pit),
or even move him to the village as a local trader; you can do this by selecting a villager npc (shift+right click as this will make them the active sim), and then use the villager to 'add to active family' (thus removing the new merchant from merchant house to live as a playable villager).
Save your game as a new file.
Hope this helps. If this doesn't work for you then describe where this advice fails, at the problematic step.

I turned Poor Maysant (npc) into a merchant. Then I found the merchant and his wife and added both of them to active family. That appears to have worked. When I click on them to go home they no longer go to the castle.
thanks for the help.

Could something like the above work in reverse? See I have the inverse problem, and rather then starting a new thread I thought I would ask here.

I was messing around with TestingCheatsEnabled and I made my Monarch's royal advisor a Spy and married her to the Merchant, but for some reason it added my Monarch to their family and now says the Market place is his home to. Is there a way to fix this with something like the above?

Since Build Master Durant seems to be permanently tied to the castle, activate him during a quest. Then add use him to add monarch to active household. That seems to be the likeliest solution.

Thanks, I'll give it a try!