The Dutch earned money, yes, and the 'mericans gained workforce, and it's the workforce part I'm interested in. The Dutch probably got no lasting economical effects but I'm guessing.
I have no idea of how much the slaves contributed to the American economy but I doubt the contribution was insignificant. Certainly the African-Americans of today will argue that their contribution throughout your country's history was anything but. I'm not arguing anything here; I'm just returning the favor by asking another hypothetical that can't be answered.
Oh, and as for tribes selling their own, it wasn't always that easy. Many tribes did not approve of slavery, at all.