There does seem to be some confusion regarding the M9 corrosion issue, vis-a-vis the replacement sensors. When corrosion was 1st addressed, all Leica could do is replace the sensor with another identical sensor/coverglass. Obviously in those cases, many of these replacement sensors also corroded. Then sometime I believe in late 2016-2017, Leica worked with the sensor manufacturer to come up with a newly designed coverglass/sensor that addresses and avoids the corrosion issue. M9's that have this newly developed sesnor/coverglass, no longer have the corrosion issues nor have to worry about it. Problem solved. Leica by sending them the serial number can let a potential buyer know if their camera has the new non cooroding sensor or if the seller has the work order for sensor replacement and it's after that certain date (late 2016 I believe or early 2017), that M9 is good to go. Leica at same time of service, provided a CLA, new leather and more.
So much mis-information is out there regarding this sensor corrosion issue.
Lastly to many, the output of the M9 raw files have something special to them. It's not every lighting/shooting situation and of course using the camera at low ISO's is advantageous with this camera. Yet when shooting both M9 and M240 cameras side by side (same lens, same subject) at base ISO, many times I preferred the output from the M9, regardless how the raw files were adjusted in post processing. Yes the M240's are a significant upgrade in terms of capabilities on many fronts, but if the end product alone is considered, the M9 holds its own and for many, far more than that. That's one of the reasons it's value and selling price has remained close to the M240. It's certainly not from the lack or shortages of M9 bodies nor nostalgic reasons. Again it's subjective and one has to consider their priorities when choosing which model digital M camera to shoot with. Same goes for the M10.