The Z4 NEVER had a good resale or sales history. I owned a 2005 3.0i and while I liked it never found it to be overly high quality, hence why I went back to the X5.

The Z4 had a 5k+ rebate for many years including the '08 as far as I know. I got mine (MSRP 51k) for the mid-40's, If I recall correctly it was 42k so a major discount.

The used ones don't seem to hold value terribly well either. I would offer 30k if it were me and even that may be a bit to high.

Remember when negotiating the kind of person who would purchase this car from you. How many people honestly could have their sole car be a 2 seater roadster? Very few. Negotiate accordingly.

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The Present:
2014 Audi Q5 TDI Prestige
The Past:
2013 Lexus GS350
2013 VW Golf TDI
2007 BMW X5 4.8i LOADED & Loved
2009 VW Jetta
2008 VW Touareg VR6
2005 BMW X5 3.0i
2005 BMW Z4 3.0i
2004 BMW X5 3.0i
2003 BMW 325i
2000 Ford Explorer Eddie Bauer 4x4