Well, here is my recent experience. I bought a used M8.2 off ebay from a seller that was legit. I checked his other auctions and past history and he clearly had been buying and selling good quality stuff for awhile. He bought an M9 and put this up for bid. I had several emails from him and ended up getting it in a buy it now deal.
The camera was in pristine condition, really, not a single mark on it. Less than 2500 pictures were taken with it. I was quite happy with the deal.
The sensor was dirty. A rocket blower did little good, maybe redistributed the dust a bit. I bought an arctic butterfly and it cleaned the sensor amazingly well.
A few weeks later, playing around with the camera, I took some underexposed shots but didn't delete them. Once uploaded, I saw the vertical line. It was reproduced in several shots. This was the weekend and I fretted about this for the balance of it. I contacted Leica NJ on Monday and they were great to deal with. With out committing to anything, they explained the process and how to get it to them. I wrote a letter explaining my situation and clearly stated that I purchased this used from an individual.
They responded a week after receiving the camera and are doing the pixel restoration as well as the rest of the checks under warranty! I really did not have high hopes, was expecting to get charged for the work but they stepped up and are covering the work. I also let them know that I noticed the coffee stain (only if the background was very light) and that is being included also.
This is my first venture into digital rangefinder cameras and what I was hoping to avoid, happened, but what I wasn't expecting, also happened. For me, the end result is good. A little down time and the expense to ship the camera off, but ultimately I will be back in a good spot. Of course, I would have preferred to have just gotten into a perfect copy and never have issues, but that was not to be. This was my "what's the worst that could happen" and the results from it.