Out of the pit of my past (and sometimes painful) experiences about buying a Minox camera: if possible, make sure the camera has its presentation box, and that the camera is in pristine or as new condition, or hardly used. This is because if the camera is accompanied by its original presentation box, there are great chances it has been sitting in it for the last 10-20-30 yrs or so. This also means that the meter has been sitting in the dark for a long time, and if so, its selenium cells decayed at a much slower rate than usual. And if the camera is not used, or hardly used, it would mean (theoretically) it was not out of the box for long periods of time.
Of course, it is entirely possible the box it's not that of this particular camera, and someone is selling a Minox B with a different box, acquired separately to this purpose. But then again, when dealing with vintage items such as this camera, one has to be careful.