OP: I know it's not completely accurate but the Private Party Kelly Blue Book can help you "estimate" approximate value.
I also agree on a private inspection - if it checks out then go for it. Keep in mind you will need new tires soon.
The E90 M3 is a Fantastic Dual Purpose vehicle. I've owned 2 so you could say I'm biased. Only vehicle I've owned where you can go out to dinner with your Wife and other Couple and the next day load up your 4 track tires/rims (fold down rear seat option) and take to the Track.
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