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You might try the W220 forum. But private party value in good condition appears to be $6k. I'm actually shocked it's that high for a 2003, but GLWS. The used car market is thin right now, so it's probably a good time to sell.
Go to carmax and carvana put in your VIN and that will give you an idea of the quick and easy sale price. Then check autotrader and search within 500 miles of your location for you exact car, these will give you a idea of the "easy" sale and the longer private parry one.