This was an era when nearly all trade was via rivers and the sea. Movement overland was so much slower and more problematic that it was generally only resorted to when there wasn't a good naval alternative.
The town was at the confluence of the Sashe and Limpopo. Such a spot would naturally control and access all trade upriver on both rivers. The fact that the town increased in size and power to the level it did (probably only about 5,000 people, but that's still a lot for that time and locale), is all the evidence we really need that this is what was going on.
So the watershed below shows roughly the area that their commerce had access to. They would have had control of nearly all the trade in the upriver (western) half of this area.
The exports they had access to that were of interest abroad were primarily ivory and gold. This would have been tied into the Indian Ocean trade routes of the era via the mouth of the Limpopo.
Here's a map showing the Indian Ocean trade routes of the era (just before, technically). That curling river at the bottom is the Limpopo.
You can see it would have been a bit of a hike up the coast to the Mwenemutapa trading port of Sofala. So any external trade probably (technically) went through there first.