Well, the colonists in the New World didn't just go over to Africa with giant butterfly nets and round up some black people. The original slaves were bought from African slavers. Between the 13th and 15th Centuries the Empire of Mali sold slaves on the West coast of Saharan Africa, and for another two hundred years after that, that area was home to slavers. The same is true of Ghana and Songhai.

In 1807, when Britain abolished slavetrading, the King of Bonny (part of what's now Nigeria) said:

"We think this trade must go on. That is the verdict of our oracle and the priests. They say that your country, however great, can never stop a trade ordained by God himself."

In Africa, it is still a problem in some areas, most notably Nigeria and Ghana, the latter of which still uses virgin female family members as compensation from a wrongdoer to the wronged family. It was only outlawed in Mauritania in 2007.

Mostly I'm just sick of people pointing the finger at white Americans for having an ugly history of slavery.

The problem today isn't just the ugly history of Trans-Atlantic slavery, it is mainly its legacy of racism, which was used as a justification for slavery. It is why black people especially in America have been perpetually discriminated. In Greek and Roman slavery, race wasn't developed as a form of identity (they took other 'whites' as slaves), rather fluild cultural concepts (non-Greek or barbarian) were used. Sorry if this wasn't the answer you were looking for.