It's actually a pretty good idea, and it can work well as a guide when done correctly. Personally I quite like them when they're structured in the form of Q&A, for example:
Why are you moving on?
What are you moving on to?
What made you choose that particular car?
What were the best bits about your Zed?
What were the worst bits about your Zed?
Anything you would do differently if you owned a Zed again?
Gives a rounded explanation and not only explains why the poster bought his or her new car, but also gives details of Zed ownership to help a potential buyer.