I am selling my 2003 Grand Am and took it down to the local car wash yesterday for a good scrub and a vacuum. I spent about $9 total and used an hours worth of elbow grease to clean the windows etc. and get it looking respectable.
Here is my view - I have a mechanic who does/did all the work on it and asked him to print me out a copy of all the service work that has been done on it. To me, and hopefully to a potential buyer, showing that the car has been properly maintained and serviced is better than showing its been detailed. $150 for the detail is about 5% of my asking price - not something I am willing to pay. As I listed it on CL yesterday, I am happy with how it looks (should a 10 year old car look perfect?) and how it has been maintained and think this will lead to a successful sale.
As other mustachians weigh in, I will be shocked if they recommend a detail vs. elbow grease at the local car wash. Good luck with your sale!