280SEBENZE is right: "a car is only worth what someone will pay for it. It doesn't matter what an estimator kicks out."
But... how do you know how to price it or where can you get an idea?
eBay motors "Completed Listings" is an excellent place to see what is REALLY happening... real auctions and transactions, on a national level.
AutoTrader, Craigslist, Cars.com all of the other big sites will show you what people are ASKING for cars... but what are they really selling (or NOT selling for?)
eBay motors completed auctions is the best place to see... you'll see actual sales with prices (in green), you'll see tons of cars that didn't meet reserve, but you get to see the high bids, even if the car didn't sell. That should give you a great idea what people are 'willing to pay'.
On eBay, go to the cars you want to look at that are for sale in your case, SL models.
Then scroll down the page, you'll see a link on the left side of the page that says "Completed Listings" click it.
It will show all the completed listings back a while... you'll see unsold cars in red, sold cars in green and you get to see the high bids, the unrealistic reserves, everything.
The truest, most Darwinian way to find the true 'value' of your car, in my opinion, is a NO RESERVE eBay auction... with a good, honest description and tons of photos.
I just sold a 2011 SL on eBay the other day... (link to auction here)
the guy lfew in this morning to pick it up. Didn't meet reserve, high bid was just shy of $70k, I called the high bidder after the auction ended. We put a deal together just shy of $89,000.
I've attended several eBay seminars put on by eBay corporate set up for car dealers, they say more than half of car sales on eBay happen OFF of eBay... meaning... a deal is struck after the auction or someone calls up and a deal is put together on the phone, that's where the whole 'car is available locally as well, we reserve right to end auction at anytime' comes in. eBay knows that. They're happy with their $50 (or $57 with reserve) listing fee if dealers list a bunch of their cars. Tought to build a feedback score that way, but you're selling the car.
I love eBay... great place to see tons of cars... you just have to know where you're buying from / who you're buying from and as always, pay for the PPI, it's aLWAYS worth it and caveat emptor.