All of this points to the camera having been used when you were told it was new. That is what would bother me the most. A battery cover like that doesn't get torn up from changing the battery once. In a studio setting a camera could be used quite a bit and still look new. I think asking for $50 is very reasonable and I might be tempted to ask for more. Test the camera first, and if you can't get the kind of refund you want, go to KEH.

Good luck.

Paul