Whilst i feel the above post is an attempt at viral marketing here is some information that may be pertinant.
In Australia data on used car prices is available from "red book" online.
Input the make model and year and the Red book website will advise on a the price range you should be dealing with. The condition of the car and features will affect the price.
Here is an example from The Australian red book for a 1991 300 series Mercedes benze.