I bought a second-hand Rolleiflex 2.8 a few months ago. It was heavily worn on the outside, but the optics were in good condition, the shutter and crank ran smoothly and the price was right. I was told by the shop staff that it was previously owned by a pro. I took it outside to load it and it promptly ate my film. On the second try it ate my film again (same roll.) Upon closer inspection, I found that one of the rails? on the back door that helps run the rollers had been bent and warped - either by repeated opening and closing, or just forceful closing. The store refunded my money, left the price untouched, wrote "junk" on the label and put it right back into the window literally five minutes later Lesson learned: ask to run a test roll through it next time.
I'm extremely wary of buying dead stock, new-in-box and/or sparkling-clean cameras. I had a friend who insisted on this, and a few times he was bitten. The funniest was when he bought a Rolleiflex T that had been sitting in a glass cabinet for years. We went all the way out into the middle of no where for a day of shooting. He loads the film, cranks it to take his first picture and the camera quickly seized up. I had to stifle my laughter as he fumed around - I though he was going to pitch the poor thing into a river or kick it into the trees
I'll buy something worn and working over a museum show piece. Cameras (lubricants, really) don't like to sit around doing nothing.