mobile.de is considered to be OK, of course a lot of scammers anywhere in the world when it comes to selling a used car.
Get a prepaid mobile phone card and only put this number in your add, throw the card away after your done with the deal, otherwise people won't stop calling you forever asking wether the car is still available and if you would sell it for 13.99€.
I only gave an email address on mobile.de, and got lowball offers from people as far away as Leipzig wanting to pick it up that day (not realizing that we'd have to pay a visit to both the zollamt and American customs before the car changed hands). I ended up selling the car via my Stars and Stripes ad to an American living in Italy. craigslist was an eye opener with the scammers who wanted to pay me via paypal and turn the car over to their 'agents'. Some of the responses were actually funny.