Humidity alone can rust edges.
Get out there with it, it's the perfect companion to the mystery. You will really like it.
Humidity and oils from your skin can cause oxidation/rust like that. I've also seen shops mount bindings for display. The shop down the street had a board with bindings on it trying to promote a last years model with last years bindings as a package price. The shop sounds like they have been responsive to you so its doesn't seem like they are hiding anything. As far as rain, sometimes business logic is absent lol.. You got a hell of a deal bro so I'd deal with it if you love that board..