Some good points in this thread. I sold my 370 in December. It was a 2009 sport package with 75k. One owner, no accidents and a motor (replaced by Nissan) that had a documented 15k on it. It needed a new differential and i got 17.5k for it privately (sold in 2 weeks). That is a 50% all in depreciation hit from my purchase price in ~5 years. This was also below the kbb at the time (~19k). The BEST offer I got from a dealer was 14k despite the trade in value on kbb being much higher.

After I sold the Z, I paid almost 20% over Kbb for an E46 M3. I did a ton of research before pulling the trigger. For m's in very good condition, this is not unreasonable. Bottom line, I know kbb and Nada are the only broad metrics we have but they really are very unreliable.