Although I am not an intense fan of aluminium bikes I am a believer in Cannondale Quality and My MTb's have taken a lot of abuse.

AS long s you get a good price for the bike and you're comfortable with not having a warranty on the bike then I'd say you could be having agood deal.

I would ask the owner a few questions like:

What type of riding do you do?
Do you race?
How often and in what category?
Tell get him talking about his commute and if possible drive along the commute route to get a feel for what the bike has to endure every day.
There may be pot holes or just generally bad roads along his daily ride.

All this wil give you a picture of what the bike has been doing and then yuo vector in his mileage and you ameka decision on the amount you'e willing to pay.