I'm an automotive tech by trade. When I run across a decent deal for a car that the owner doesn't want to spend money on, I usually make them an offer and buy the car. And they are usually really bad inside and out. They just didn't give a s--t when they owned it.
After I fix what's wrong with the car, i bring it home and spend the next month detailing the pis out of it. I know, I know...a month seems like a long time, but I work 10 hours a day, and I'm married, and I have a kid and friends and side work....so sometimes it might be a little longer.
Next up, new brake pads, oil change, new belts, new shocks or struts, and in some cases, a new windshield.
After all is said and done, I put it up for sale, and always ask more than Kelly Blue Book.
When people come to see the cars I sell, they ALWAYS comment on how smooth the paint is, how their are no swirl marks, how clean the rims look. The interior is spotless, leather smells like leather. Fabric is clean crisp. Carpet is very clean (or as clean as 3 days spent getting it to that point) new floor mats if needed.
Last year, I sold a 2003 Honda Civic with 198k miles on it for $3,700. I spent maybe $700 on it, and that included the car !!
People care about 3 things here in Texas:
1) does the a/c work
2) does it look nice
3) does it drive good
IN THAT ORDER !!
My wife was amazed that I go so much for that car that she called a piece of crap. And the girl who bought it said it was the cleanest car that she and her mom had driven in 2 weeks.
People associate a clean car with a well taken care of car.
So short story long....YES.... Spending time or money getting a car detailed before you sell it is an investment in your bottom line. You can command more money than if it was trashed out and dirty. You come across as someone who took pride in their ride. And , believe it or not, you seem more trustworthy.
It's worth every dam cent.