I haven't put any film through the camera yet. I picked it up at the post office on my way to work. I'm just holding the camera up to daylight with the back open and I can tell it's happening. I'll shoot some photos at noon with the camera and develop/scan them tonight.

The resulting image would look the same as if you were incorrectly using speeds faster than your flash synched to though. But no, it's not flash related.

I doubt you can tell by looking at the shutter firing so the seller probably didn't know. Probably dried up lubricant.
You can tell. Just hold your camera up (no lens in place) to some light and fire it with the back door open. You can use an all white screen on your computer monitor too. It's pretty easy to see. However, I'll take photos to prove what I'm seeing already