When looking at a C check the cab over very carefully, any bulges on the sides or front inside and out means water damage, tap the underside, the noise should be the same sharp tone, any dullness indicates damage from water intrusion, take a good flash light and look into the corners of cabinets at ground level any fine sand where they did not clean that looks like river silt, it probably was a salvaged unit from a flood or hurricane.
If you are not completely satisfied walk away, do not buy a can of worms, there can also be water damage around the vents and the rear cap like on the front, also push up on the ceiling it should all be firm not mussy.
Good luck.
navegator