Be aware that there has been a HUGE spike in used car prices recently due to a number of factors creating the "perfect storm" for the used car market:
-Recession in '08 caused people to quit leasing cars = much less used cars entering the market today than normal.
-Component supply (plus actual vehicle supply) from Japan disrupted due to tsunami
-People are keeping their cars longer because money is tight.
-Cars last longer.
-New vehicle loans at reasonable rates are only being offered to people with extremely good aggregate credit ratings.
Expect to pay at least 24-25k for that car unless the seller isn't paying attention.
If you're interested read:
Used cars set to become scarce, more costly due to recession - Jun. 21, 2011
...and here's KBB from a private seller:
2009 Nissan 370Z Coupe 2D Trade In Values, Reviews - Kelley Blue Book
~26k pristine
~24.5k good (most likely)
~22 fair (if it has had 'issues')