A little rust on the armature or field is not an issue. I've had them that were really totally rust covered, I cleaned it off with ScotchBrite and all was fine. That's unprotected metal, and they rust pretty easily.
That being said, I kinda' agree with the "send it back" reply. Without actually seeing it, it's hard to evaluate if it's a minor issue or a disaster waiting to bite you.
EvapoRust is your friend. 
