To return to the question.
If you are comfortable dealing with potential buyers yourself, you can post it in the For Sale section here, try an ad in the MBCA Star magazine, or list it in the Mercedes Heritage classifieds, or even put it on e-Bay. IMO, the keys in these venues are good pictures, a description of the car and its history that is factual and believable, and a realistic price.
On the other hand, a place like Cosmopolitan will handle the customers and give your car exposure to a bigger audience. Being in the business, they may be able to get a better price for the car so you might break even or better than selling it yourself.
I have wandered around Cosmopolitan a couple of times and they seem to be nice people and they have had nice cars on the floor. They have been in business for a long time so they must be doing something right. However, they are not known for their Mercedes expertise. You can always google them and ask them for a couple of references.
Like Rolf-Dieter said, if you post some pictures and details about your car, we may be able to give you some guidance on pricing.
Good luck,
CT
