I would try to get an idea of how many shutter actuations it has before you buy it. Also, check for vertical lines at high ISOs, rangefinder misalignment, and back/front-focusing. I've had all of those issues with a used M8 I purchased in January--issues I'm still trying to resolve after twice having already sent my camera in for service.
As for price, look around on the forums like this one, GetDPI, etc. as well as eBay to get a sense of what they're going for. Good luck!