It honestly wouldn't bother me tbh. If it was a 3-4 year old car then yes it might concern me but it is getting on for almost 10 years old now.
Sometimes people buy these cars then realise they can't afford to keep them for various reasons like: running costs (petrol/insurance/service bills), change of job, loss of job, need more seats etc etc.
Also some people don't seem to keep performance cars for very long either. They sort of buy a car just to experience it, tick it off their list and then move on to the next whim of fancy.
I'd be more interested in the service history if I was you. Just check to make sure everything has been done dependent on the mileage/age etc.
Service schedule here: http://www.350z-uk.com/topic/31110-service-schedule-for-a-350z/
Also worth having a good read of some of the threads in this section: http://www.350z-uk.com/forum/18-350z-faq/