I guess it's possible the remaining ones are still packing, but somehow I doubt it. In my experience, at least, they say they're leaving, they all pack up, and when it's packed, that's when everybody walks off. IOW, nobody walks out the door until it's all packed up.
Sometimes I wish merchants WOULD go home because they ask for FAR too high profit margins. I mean multiple grand profits from a single exchange...
The only time I really ever had a problem was in the 2D version; sometimes I just didn't have enough to trade that first autumn. By about year 3 or 4, I'm usually giving obscene amounts of profit without blinking an eye, either from food or cloth images or goblinite or crafts or green glass discs. Just depends on what's going on.
Of note, if you have an inexperienced broker (which you generally do at the beginning unless you sink embark points into it), he's going to think your stuff is worth more than the traders do - he's going to be enthusiastically saying "Here's a 20-dorfbuck dolomite mug" and the traders are rolling their eyes and thinking "geez, that's worth 5 dorfbucks at most. That fortress in the next mountain range has a way better broker than this chump. This is gonna be a pain."
For my first year, I generally plan on giving the traders a really good profit margin. If that means I can't buy as much, so be it. Better than having them pack up and leave in disgust - I don't like stealing the goods from them, personally.
Mechanisms trade relatively well and skill up the mechanic at the same time. Prepared meals command high prices. Green glass discs are another awesome trade good.