I've used online service before to find a car I wanted.  I'm trying to remember the exact service.  Basically it allows you to search dealer inventories across the country.  I wanted to buy a vehicle that was from a state where they don't salt the roads in the winter, and was able to find one with the options I wanted, low miles, and no rust.  It worked quite well, and a trip to VA with a trailer and back later and I had our FJ Cruiser.  I paid a bit of a premium, but condition being everything it was well worth it. 

I've only used craigslist in the past, sold 3 cars at asking value but they were old used cars that had served their purpose.  Autotrader ask for $100 listing fee but no fee on sale price.  That's about the going rate.  

Something to keep in mind... if you go through an online site they almost all are required to retain taxes if you do the financial transaction through them.  Yea it's safer and more convenient then working in cash, but if you do the transaction in cash you can skip all that mess.

Obviously, for high value cars you're not going to get a lot of people with suitcases of cash though.

I use KBB to get the info of value and cost of what I want and what I have to get rid of.
I DID learn you get a better "deal" at the dealership when you pull the trigger toward the END of the month and/or quarter.
I 'might' get a little more $$$ if I sold the car myself butt.....I don't like    "strange-folks"   coming to my  house/home/place/crib/ranch whatever. Let the dealer handle it and get a little less.
I'm big on NOT financing stuff either..........Save up until you have enough$$ to get what you can afford. Not sure if this  effects the end price butt..........datz-how-I-roll
6sal6