Q: If someone uses the power of Stolen Goods, who gets to decide what the other person trades?

A: The person with Stolen Goods is the active player (i.e. it’s their turn), and they get to decide what they take from the other player when they give away the Stolen Goods to make this forced-trade. Of course, they can only trade with Keepers already on the table, not Keepers in the other player’s hand.