Test it out at the shop. Run it through every ISO and every setting with a lens of your choice and an SD card that you own. Underexpose several frames at every ISO. Underexpose by more than 3 stops. Do some long exposures. You'll be looking for hot pixels and vertical lines that come from those hot pixels. The lines look greenish or maybe reddish. Some of the older sensors can also show a different mapping in which the right and left sides may look different. If this is obvious, then turn it down; if you can see it but it's not too noticeable, it's a normal thing that can bee seen rarely.
If the seller doesn't want you to test it that much, absolutely don't buy it. Warranty or not, his warranty won't be honored by Leica and you may put yourself at risk for a very expensive repair if there is a loophole. M8 repairs start at about the cost of a EXC+ M3 body, so the warranty is a big deal.
Phil Forrest