As far as I know they are not packing up their goods, they are just standing there. The outpost liaison has already left. If they go insane and I have to salughter them, will I be at war withy my parent civ?
EDIT: Thank Armok they're leaving, I have no idea why it took them this much time.
The way to know if they are packing up their goods is to look at the items inside the trade depot (with "t"), just as you do with other buildings. You would see the list of items shrink over time.
It probably took them a long time because of what you traded them. Did you give them a thousand loose items that weren't in bins? Or did you trade entire bins all at once? Just like how bins make things easier on hauling jobs inside the fort, they make it easier on merchants, who always pick up "one item" at a time.
Merchants are chilling in the Trade Depot, while they should have left some time ago. I even got the message that they have embarked on their journey, even though they did not.
Quick note: have you watched much trading in general? You get that message when they start to pack up, not when they finish. They always hang around after that message for awhile.
For some reason, once you get the 'embarked on the journey' announcement, if you open up the designations menu, 'd', and hold down the mouse button, they'll pack up their goods quickly without game time passing.
I don't know the details, but this has to do with several things in the game advancing on mouse-click despite the pause. They just break the laws of time. Job changes based on military activation are one I often see, and I believe pet adoption is another. In short: beware of abusing that until someone more experienced explains it better.
--Rexfelum