What happens if I target Zada, Hedron Grinder with a spell and then donate it to an opponent?
Zada's ability will still resolve, and you will still be the controller of that ability, so donating Zada will not make any difference.


  112.8. The controller of an activated ability on the stack is the player who activated it. The controller of a triggered ability on the stack (other than a delayed triggered ability) is the player who controlled the ability’s source when it triggered


Emphasis mine. Zada's ability triggers as soon as you finish casting Traitor's Roar. It then goes on the stack, as an ability controlled by you, before you get a chance to respond with something like Bazaar Trader's ability.

If you did respond with Bazaar Trader's ability, then after that resolves, Zada's ability would still resolve like normal, and you would still be the one affected by it. You would be the controller of the copies of the spell, and they would target your creatures.


  111.2. A spell’s owner is the same as the owner of the card that represents it, unless it’s a copy. In that case, the owner of the spell is the player under whose control it was put on the stack. A spell’s controller is, by default, the player who put it on the stack. Every spell has a controller.