How do you stop the thief on poptropica?

At night you have to go hide by the bushes near the entrance to the auctions. You will see her leave then follow behind her, hiding behind object when you go around the island she enters the shop where you buy the sail. Run in after her and she can't escape. Talk to her and then she will change her ways AND give you a bottle. (i am not sure about the bottle part but i think it i true).

Yes you do get a bottle from her. but be careful about how you answer her questions because otherwise she will leave.