I think the only downfall with our rating system is that not a lot of people will not give negative ratings for the fear of being confrontational. That's what I notice anyways.
So whoever did it, has probably done something similar in the past, and whether the other trader(s) were too nervous, got ninja'd like you, or they feigned naiveness.
Personally, the person should know the number 1 rule is to always ask the owner of the island if they're unsure of the answer. Just trading and being here for a week screams that, lol.
I'd love to use the pearls as decoration but I don't want someone yoinking it. I'm sorry that happened during a nice event. It sucks there's really no way of curbing it in a simple fashion without pulling out the fences or hiding them in storage.