1)
When did they sell Bnei Yehudah and Bnei Yerushalayim?
1.
Radak #1: When they were in their land, you stole their children and sold them to Bnei Yevanim.
2.
Radak #2: In Yisrael's Galus, they came with [Yisrael's] enemies, and took the children and sold them to Yevanim.
3.
Malbim: At the time of the Churban.
2)
What do we learn from "Lema'an Harchikam me'Al Gevulam"?
1.
Radak: They sold them to Bnei Yevanim, who were far from Eretz Yisrael, in order that they would not return to their land.
2.
Malbim: This is worse than selling them for your profit, to benefit from the money. You sold them to a far place amidst hatred. Therefore, I will repay you Midah k'Neged Midah!