Farzoi's chief racket and his northern boys.

First of all this:

http://www.princeton.edu/~pswpc/pdfs/scheidel/050704.pdf

Cf. especially the text to footnote 48.

Then have a look at Strabo (Book 11, section 2, paragraph 3).
It appears that slave trading was one of the Eurasian nomads' chief occupations. And Greek city states were intermediaries. The Scythian fortresses of the Lower Dnipro fulfilled the same basic function.