I have a new question, how can I know if a low mileage camera has the original sensor or if a replacement ? Because there is another local seller with a 5dmk2 with just about 5k shutter actuations...he said is the original sensor but how can I know if is true or not ?
No way to know for sure, but having the sensor replaced is fairly rare in the realm of failed components.
The question I would be asking is WHY it was replaced. If it was because of external damage like a fall or water, I wouldn't buy it. Sensors do fail, but it is not that often unless you put it in front of a laser.