Look around with your Explorer before choosing to buy land from a tribe.  As you discover tribal capitals consider whether this would be a good place to buy land. Here are some factors:

1. It is better to buy land from a Tribe you have not already attacked. Tribes you have attacked dislike your Empire so you will not benefit as much from overseas profits. The amount of overseas profits on a commodity sale is  divided between the Tribe (or Neutral State) and the Great Power that bought the land. As the relationship improves, the Great Power receives a higher percentage on each sale.

2.  If you see small colored flags on tiles around the tribal capital it means that another Great Power already owns that land. You do not want to waste the money for an embassy only to learn that the land is no longer available.

3.  Finally, consider what resources you need. Buy land that produces resources your Empire lacks.  You can tell what sort of resource is produced in a tile by holding your mouse over the tile and reading the text at the top of the screen.  If you info-click on a tile you can obtain more detailed information. Remember that for this episode you are concerned only with tiles adjacent to a tribal capital containing a development.